Add Scale-Invariant Heat Kernel Signature classes

[gviga]

This Pr add a scale invariant version of the Heat Kernel Signature to the possible descriptors to use.

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Codecov Report

❌ Patch coverage is 41.17647% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
geomfum/descriptor/spectral.py	41.17%	10 Missing ⚠️

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[GiLonga]

Hello @gviga, I'm starting to review this PR and I already have a question about the two new classes.
The call methods are basically identical, and the init methods too. Would it not be enough to add a method in the base classes that returns the scale-invariant descriptors instead of creating new classes?

I understand the object-oriented approach, but in this case I'm not sure it is trurly useful.

[gviga]

Hi @GiLonga ,
Thank tou for reviewing this PR. I'm not sure i got your point:
Are you referring to the fact that both ScaleInvariantHKS and LandMark scale invariant hks have the same call function?
About the object-oriented approach: we need a scale-invariant-hks class with a call function that computes the descriptor, so it is compatible with the API of any descriptor.