TDVP small refactor

[kbidzhiev]

I've simplified logic in TDVP, making a first step toward generalisation.

• _swipe_direction, _sweep_index and _timestep_index renamed because they are internal variables of the class
• _swipe_direction, _sweep_index and _timestep_index have default values and its reflected in a default constructor
• Sweep direction and MPS node indexing are logically not coupled, its now reflected in the code

[a-quelle-pasqal]

LGTM