* Add SparseVector
temp
* Add gtest
* Some reworking of the sparse concept
* Change type of M from int to size_t
* Add const modifier
* Add needed declaration for accessing elements of _indices
* Add norm_squared, norm, longerThan
* Add test for all sparse vector functions
* Add missing const to slice's norm_squared, norm and longerThan
* Construction from Vector<M> and carray[N]
* try to fix ci
Co-authored-by: Julian Kent <julian@auterion.com>