Unify lagrange implementations

This merge request implements a new class LagrangeSimplexLocalFiniteElement that unifies all existing implementations of simplex Lagrange LFEs with compile-time dimension and order. It also replaces all existing implementations by the new one in a fully backward-compatible way. The old implementations are not formally deprecated, but the doxygen documentation now mentions that their use is discouraged. The old internal ***LocalBasis, ***LocalInterpolation, ***LocalCoefficients-classes are completely removed. I consider them implementation details not subject to the usual API stability rules.

The advantages of the changes in the MR are vastly shorter code (a single file instead of an entire directory tree), and a more consistent naming of the different simplex Lagrange implementations.