Gear generator classes (#19)

* Create gear_generator.py

* Add bevel gear

* Update gear_generator.py

* add crown gear

* Create rack_gear.py

* add docstring, improve organisation

* changed imports

* add readme and setup files

* moved to module dir structure

Implemented the same changes as @marcus7070  did for the more_selectors plugin

* update functions calls

* testing import solutions

* made plugin functions work

* setup test functions

* Update .gitignore

* add working tests

* add auto link of methods in __init__

* change function names

* Update cutter_objects.py

* add register fct to modules

* add imgs for readme

* finalize PR version

* Update README.md

* Update test_gear_generator.py

* Rewrote the plugin in OOP

I rewrote the plugin using OOP. For now there is only bevel gear and spur gear, I removed all the other ones cause they werent mature enough

* correction of error in rotation angle

change twist angle from radians to degrees

* Update test_gear_generator.py

modified tests to handle the update in twistangle

* Update test_gear_generator.py

modify precision due to difference with ubuntu 18.04

Trying hacky stuff and triggering the test

* Revert "Trying hacky stuff"

This reverts commit a37287b.

---------

Co-authored-by: Marcus Boyd <mwb@geosol.com.au>

plugins/gear_generator/README.md

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+# Gear generator
+
+This plugin provide classes to create various gears. 
+As for now you can create these gears (all the gears are involutes):
+* Spur gear
+
+<img src="images/straight_gear.PNG" width="600"/>
+
+* Helical gear
+
+<img src="images/helical_gear.PNG" width="600"/>
+
+* Bevel gear (straight and helical)
+
+<img src="images/bevel_gear.PNG" width="600"/>
+
+* Bevel gear system (straight and helical)
+
+<img src="images/bevel_gear_system.PNG" width="600"/>
+
+
+## Installation
+
+To install this plugin, the following line should be used.
+
+```
+pip install -e "git+https://github.com/CadQuery/cadquery-plugins.git#egg=gear_generator&subdirectory=plugins/gear_generator"
+```
+
+
+## Dependencies
+
+This plugin has no dependencies other than the cadquery library.
+
+## Usage
+
+To use this plugin after it has been installed, import it and create Gear objects
+```python
+import cadquery as cq
+import gear_generator
+
+module = 2
+nb_teeth = 12
+width = 8
+gear = Gear(module, nb_teeth, width).build() #Instantiate a gear object and call it's build method to get the gear in a cq.Workplane
+```
+<img src="images/readme_example.PNG" width="300"/>
+
+Below is the list of implemented gear classes :
+```python
+Gear(args)
+BevelGear(args)
+BevelGearSystem(args)
+
+#You can get info about the parameters by running 
+help(BevelGear)
+
+Help on class Gear in module gear_generator.main:
+
+class Gear(BaseGear)
+ |  Gear(m: float, z: int, b: float, alpha: float = 20, helix_angle: float = 0, raw: bool = False)
+ |
+ |  Base gear class
+ |  This class stores attributes that are shared by any types of gear
+ |  Other gear classes inherit from this class
+ |
+ |  Attributes :
+ |      m : gear modulus
+ |      b : gear tooth facewidth
+ |      z : gear number of teeth
+ |      p : gear pitch
+-- Suite  --
+```

plugins/gear_generator/gear_generator/__init__.py

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+from .main import *

plugins/gear_generator/gear_generator/helpers.py

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+from math import cos, sin, radians, acos
+import cadquery as cq
+
+def involute(r: float, sign: int = 1):
+    """
+    Defines an involute curve to create the flanks of the involute gears
+
+    Args:
+        r : Radius of the involute (for a gear it's the pitch radius)
+        sign : 1 or -1 , to draw the involute in positive or negative direction      
+
+    Returns:      
+        x,y -> tuple() : 2-tuple of x and y coordinates in space
+    """
+    def curve(t):
+        x = r*(cos(t) + t*sin(t))
+        y = r*(sin(t) - t*cos(t))
+        return x,sign*y
+    return curve
+
+def spherical_involute(delta, delta_b, R):
+    """
+    Equation of the spherical involute that lies on a sphere
+
+    Args:
+        delta : the function variable, goes from the gear root cone angle to the gear tip cone angle
+        delta_b : angle of the base cone
+        R : radius of the associated sphere
+
+    Returns:      
+        x,y,z -> tuple() : 3-tuple of x and y and z coordinates in space  
+    """
+    theta = acos(cos(delta)/cos(delta_b))/sin(delta_b)
+    x = R*cos(theta*sin(delta_b))*sin(delta_b)*cos(theta) - R*sin(theta*sin(delta_b))* - sin(theta)
+    y = R*cos(theta*sin(delta_b))*sin(delta_b)*sin(theta) - R*sin(theta*sin(delta_b))* cos(theta)
+    z = R*cos(theta*sin(delta_b))*cos(delta_b)
+    return x,y,z
+
+
+
+def rotate_vector_2D(vector: cq.Vector, angle: float):
+    """
+    Rotates a 2D cq.Vector `vector`by an angle of `angle` in degrees
+    """
+    angle = radians(angle)
+    x = cos(angle)*vector.x - sin(angle)*vector.y
+    y = sin(angle)*vector.x + cos(angle)*vector.y
+    return cq.Vector((x,y))