Added function for generating sparse interpolation matrices

[brownbaerchen]

This PR adds a simple function for getting sparse interpolation matrices using the barycentric Lagrange interpolation that is already implemented in qmat.
It loops through every point you want to interpolate to, gathers the k nearest neighbors on the grid you interpolate from, and puts the interpolating coefficients in a line of the interpolation matrix. Afterwards, interpolation of order k can be achieved by SpMV.
For certain grids, the distances between nodes to be interpolated to and their k nearest neighbors on the grid you interpolate from repeat for multiple entries. In this case, this function will simple copy existing interpolating coefficients rather then computing them from scratch.

Tests are included that make sure the interpolation is of order k by testing that a polynomial of order k is interpolated exactly, whereas a polynomial of order k+1 is not, unless both grids match, in which case the interpolation is always exact.
The latter case is needed to test the reuse of interpolating coefficients.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c252dba) to head (dc62dde).

Additional details and impacted files

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+ Hits          1408      1425   +17     

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[tlunet]

Thanks, looks neat 🚀