libs/libseife [FEATURE]: add M3 to tide removal in C-version
revision of tide removal code: - both versions (C and Fortran): do case of nstep=1 explicitly seife by default averages over serval samples and uses the tidal hamonic as well as its derivatives for corrections; this appears to have saved computation time in the very early times but is of no relevance nowadays; by not averaging the code (and the computation) becomes much more straight-forward and less error-prone - C-version: add M3 frequency The number of frequencies was fixed to 6 in the original version; to allow for the addition of frequencies, the dimensions of arrays now are det by C preprocessor macros; the function seife_gauss is modified accordingly to allow for a larger system of equations