Added generic Lagrangian interpolation.

I really don't know why, but I never tested
using the definition of the Lagrangian interpolator.
It is much faster than using the functions lg*j.
I am thinking of removeing all those functions. For now,
I implemented the generic `lagrange` function and the
corresponding `interp_mat` function to calculate the
interpolation matrix.
I decided to remove the functions ig*j* and lg*j*.