WIP : Restructuring the vector framework for sympy.physics

Strictly a WIP.
Will take parts from essential.py and functions.py in sympy.physics.mechanics, and bring them one level up into sympy.physics (leaving only the core mechanics parts in the mechanics module) - to make a new 'sympy.physics.vector' module
The namespace of sympy.physics.mechanics will not be changed, to avoid compatibility issues.