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

[DavidLapous]

Commit 80c699c 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