Add Exercises 11.7, 11.8.

This renames Ex11.7.py and adds Exercise 11.8 to the file.
Improve calculation of partial velocities.

Express velocity in desired frame before substitutions when calculating
partial velocities.
Fix inertia torque calculation.

Verify that the angular velocity/acceleration terms are instances of
Vector. This commit will be obsolete with this PR:
https://github.com/sympy/sympy/pull/2199.
Rename inertia torque function in util.