Multiparameter simplextree, C++ part.

[DavidLapous]

Cpp part of PR #817. List of changes :

• new is_multi_parameter flag in the simplextree's options to deal with filtration values that behave as array-like instead of float-like.
• Filtration values are now as const ref to prevent array-like copies
  • This implies adding static members for trivial filtrations : null_value and inf_
• new member filtration_mutable to have a non-const access to the filtration.
• For multiparameter filtrations, deactivated the assign behavior of the insert to prevent unwanted behaviors (and stay 1-critical). The filtration property is no longer maintained.
• New array-like structure as Filtration_value, child of std::vector with numpy-like pointwise operations, in Simplex_tree/multi_filtrations/Finitely_critical_multi_filtrations.h
• The Simplex_tree/Simplex_tree_multi.h contains the options of a simplextree multi, functions that can convert a 1-parameter simplextree to a multiparameter simplextree and vice versa, aswell as useful functions to push filtration values on a discrete grid (for discrete computations.
• I also copied the unit tests of the simplextree to apply them to this new structure.

Feel free to move files/functions if they don't follow Gudhi's standard, or commit if its moral changes.

[DavidLapous]

I'm not sure why the ci Tangential complex test fails on git, I cannot reproduce on my laptop & it doesn't fail on the other pipelines

[DavidLapous]

@VincentRouvreau I've added your tests to the end of the test file, for the NaN and I had add some checks in the multi filtration file so that the behavior of NaN and inf so that inf and nan filtration values have a proper behavior

• NaN comparison is always false
• The push_to method ignores NaN values
• inf values behave as expected.

Furthermore, thanks to your test (Merci !! 😉) I noticed that there was a bug on the make_filtration_non_decreasing in the multiparameter case, which sometimes allows non-increasing values of simplices, that's why I had to change this line

https://github.com/GUDHI/gudhi-devel/blob/a89ff98f87eb6614cdfe8f3fcd8560319a008d71/src/Simplex_tree/test/simplex_tree_multi_unit_test.cpp#L933

I've only changed this line and the last test, I hope it didn't break the spirit of your test

[DavidLapous]

Changes since last time:

• Finitely_critical_filtrations renamed to One_critical_filtration
• Added a K critical filtration alongside previous one, for the moment defined as a vector of vector, but it may be better to have it as a contiguous one : if the number of parameter is known, a np.array_split(contiguous_stuff, num_parameters) should do the trick.
• removed some functions that are not necessary yet