Improve efficiency of HLegendre-computation

The computation of the legendre polynomials is relatively slow.
It might be a good idea to hard-code the first polynomials and use the numeric-extension for the really-higher-order polynomials.
Since the polynomials are soft-coded and computet automatically, it should be easy to set up a c++-code automatically using Fesslix.

After that the function computing the polynomials should be inlined. Note however, that this function is needed a lot of time. Therefore, the speedup should be seriously compared to the size of the executable.
Moreover, if the function is inlined, it won't be clear how much time is spend in the function within an execution.