Improve interpolation interface

[IgorBaratta]

• Currently basix only provides the interpolation matrix which has shape shape = [Npoints, ndofs * value_size].
  The interpolation data for any v in value_size is not contiguous, and the number of points can be quite big for some elements.

• Other finite element functions such as map_push_forward work with tensors of dimension 3 (eg: const xt::xtensor<double, 3>& U).