Adding Discrete Vector Field Filters

[finkentA]

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This Pull Request adds 3 new filters and 1 new class into TTK which compute and work with Discrete Vector Fields given an input piecewise linear vector field. The components are described as follows:

Filters:
DiscreteVectorField(namespace: DCVF)- Similar to DiscreteGradient as it generates a discrete vector field from triangulated input vector field and displays the critical points and vector glyphs.
TopologicalSkeleton- Similar to MorseSmaleComplex as it uses the DiscreteVectorField as a backend and produces the critical points, separatrices, and segmentation of the given input field. Also allows for simplification of the discrete field using the VectorSimplification class.
VectorWeightCurve- Uses an input vector field, produces a table similar to PersistenceCurve in that it generates a curve for simplifying the discrete vector field for varying weight thresholds and the number of critical points, along with optional, additional information like whether sources(critical dimension=2) or sinks(critical dimension=0) where removed and the number of orbits being added or removed during simplification.

Class:
VectorSimplification- Class that handles the simplification of the DiscreteVectorField (similar to DiscreteMorseSandwich, but actually performs the simplification on the discrete field to determine the pairs).

[julien-tierny]

hi @finkentA ,
thanks for your PR.
for your information, if you want to adjust your PR (to make it pass the CI tests for instance), you do not need to close it (and re-open it later). you can simply push commits to the branch from which you opened the PR and the PR will get updated automatically. I hope it helps.
Best,

[finkentA]

Alright, thank you for letting me know. I will do that next time.