add factory functions for fundamental discrete symmetries

Closes #5 (closed) and #4 (closed). Should be merged on top of new_model.

• Factory functions for discrete symmetries: time_reversal, particle_hole, chiral, inversion, rotation, mirror.
• Better representation of PointGroupElements and ContinuousGroupGenerator, using _repr_pretty_ and _repr_latex_. Remove print_PG_elements and update example notebooks.
• Slight change to the internal representation of PointGroupElements, allow mixing integer tinyarray with either sympy matrix or floating point tinyarray in the rotation part, but do not allow mixing the latter two representations. This removes the need to have two different representations for simple operators. Optimize multiplication by caching.