[Optim] change vectors of "heap stuff" into tensor structures

[DavidLapous]

Commit 80c699ce994722ab080c90ea6ad7762cdf594fb5 suggests (with internal benchmarks) that non-tensor (or vectors of stuff in the heap) structures can induce significantly slower performances. Some structures could hence be changed to ensure contiguous stack structures:

• [ ] replaces vectors by small vectors for filtration values (with different capacity, chosen during runtime in python) -> TODO: benchmark this
• [ ] replace the multifiltration structure by the one used for degree-rips-like filtrations when 2 parameters
• [ ] use 2d tensors for multicritical filtrations
• [ ] Optimize current tensor implementation