Fix "Code is unreachable Pylance" warning for NumberPlane

[TurkeyBilly]

Motivation

• After initializing a NumberPlane object, the code area below will be marked as "unreachable" by Pylance
• This grey out the code and disables syntax highlighting

(When I was writing the code)

(In the source code)

• Meanwhile, when clicking "show definition" for get_x_axis() or get_y_axis, it will jumps to abstract methods under CoordinateSystem

• How to disable "code is not reachable" warning and greying out the code? I find this feature to be unreliable quite often. microsoft/pylance-release#248 (Reference)
  This warning was due to

def get_axes(self):
    raise Exception("Not implemented")

however without adding an abstract class decorator

@absrtactclass
def get_axes(self):
    raise Exception("Not implemented")

(Jumping was due to the fact that get_x_axis() and get_y_axis are not overrode again under Axes class)

This pull request aims to improve the structure and make it convenient especially for Pylance user writing and tracking codes

Proposed changes

• added @abstractclass for two abstract methods
• overrides get_axis, get_x_axis, get_y_axis, get_z_axis so that when using these/writing these methods, Pylance and users can track back to re-implemented methods under Axes class instead of CoordinateSystem where they will find nothing but a raise Exception

Test

Tested by initializing a Numberline object and no error occurred.