modify `SplitMix` use of `_state`

[schmouk]

• for it not to use _state value if zero.
• modify SplitMix64 initialization step as validated in test directory (no more calls to operator() in __init__().
• optimize implementation of SplitMix64.__call__() in all versions of Python (see Python3.13).

[schmouk]

_state is no more used as being set to value 0.
TestED tested.

[schmouk]

No more uses operator () at construction time (conforms to c++ version of PyRandLib).
TestED tested. Still to be validated.