first steps to removal of Point

[InnocentusLime]

I did the first steps to remove the Point (solving issue #502)
As result all the PointN structs were removed (and the point.rs file was removed too) and replaced with VectorN. VectorN had EucledeanSpace implemented too. Some associated types and methods were removed for simplification

1. Diff, Scalar in EuclideanSpace
2. dot, from_vec and to_vec for EucledeanSpace
3. to_homogenous and from_homogenous were added to Vector3 for convenienve

The library seems to compile in my case. Most likely the change is too radical and depreacating should be considered instead

I wasn't sure about some methods in Transform, so I left them intact

[nstoddard]

Thanks for working on this! I still think we should postpone this until 0.19 and maybe mark Point etc as deprecated in 0.18. Unfortunately this will require some significant changes to pretty much every dependent crate, so I'm not sure whether it's feasible to remove Point entirely.

If we're going to simplify the API like this, we might also consider removing some of the traits like VectorSpace and EuclideanSpace, and moving the methods on those traits into the types that implement them. I'm not sure how many breaking changes are acceptable, though.

[nstoddard]

Points and vectors have different representations in homogeneous coordinates (vectors have a w coordinate of 0 so they're not affected by translations, points have a w coordinate other than 0 (often 1)), so these methods should probably be called point_to_homogeneous etc or something similar, or made private.

[nstoddard]

This needs to divide by the w coordinate.