Use abc.ABCMeta for Geometry class

[wkentaro]

To avoid expected but not implemented methods like apply_translation of
PointCloud.

[coveralls]

Coverage decreased (-0.4%) to 83.534% when pulling 3d30ce1 on wkentaro:use_abc_geometry into ed936fc on mikedh:master.

[coveralls]

Coverage decreased (-0.07%) to 83.887% when pulling 29afcb3 on wkentaro:use_abc_geometry into ed936fc on mikedh:master.

[mikedh]

Great idea!

I might add a comment in the top of the Geometry object explaining why we're using a metaclass:

The `Geometry` object is the parent object of geometry classes, including:
Trimesh, PointCloud, and Scene objects.

By decorating a method with `abc.abstractmethod` it just means the objects that 
inherit from `Geometry` MUST implement those methods. 

One other thought: apply_translation, apply_scale can actually be implemented in the Geometry object as they just generate a transform and are the same everywhere, they just call self.apply_transform that needs the per- object implementation (it could be the abc.abstractmethod)

It looks like it needs some tweaks to pass in Python 2, there's a discussion here, it looks like the cleanest solution is (IMHO):

import sys
import abc

if sys.version_info >= (3, 4):
    ABC = abc.ABC
else:
    ABC = abc.ABCMeta('ABC', (), {})

class SomeAbstractClass(ABC):
    @abc.abstractmethod
    def do_something(self):
        pass

[mmatl]

@mikedh Another option is using six, although it adds another (very light) dependency for Python 2/3 interop. Then, it would just look like:

import abc
import six

@six.add_metaclass(abc.ABCMeta)
class SomeAbstractClass(object):
    @abc.abstractmethod
    def do_something(self):
        pass

I think that should work.

[wkentaro]

I updated. I avoided to use six module since we only need that module only for handling ABC class.
We may use six if we also have another case we need it.

[mikedh]

Looks good thanks! Yeah including six at some point may be the way to go if we run in to something particularly annoying. It's MIT licensed single- file module so it won't mess with the minimal install if we just include it as trimesh/six.py. That being said it hasn't been necessary yet, and this fix looks good.