imagefusion: Parallelize (E)STARFM directly with OpenMP

Parallelizer may not be used anymore with STARFM and ESTARFM. They are
not parallelized using OpenMP directly, which is more efficient and
easier to use.

This led to some documentation changes, since the mainpage showed how to
parallelize STARFM. Actually all included image fusion algorithms are
now parallelized internally. So Parallelizer is left for custom fusors
only and the documentation shows how to do that now.