Adding `inertia_tensor` property method.

The `inertia_tensor` property returns the inertia tensor as a
3x3 numpy array.
Updating translation and rotation transforms.
adding `scaling_matrix` function to _transforms module
updating docstrings
Large rewrite of sknano.core.math._transforms module.

Rewrote transformation_matrix function to implement
Vector and Point objects in calculations.
updating docstrings
updating core math test modules
implementing updates from transforms module for rotation and translation methods
tidying up code
adding core subpackage for physics calculations
adding physics subpackage to __init__ module docstring
adding doc sources for core.physics subpackage
Tweaking rotate function in transforms module.

If transform_matrix if not None, check if it is 3x3. if it is,
create new 4x4 transform_matrix filled with elements of
3x3 transform_matrix.
adding property method to compute the atoms centroid coordinates