SW-235: added storageWriter and storageWriterVersion keys to metadata for the various dataproduct types

SW-72: process review: moved mail generation to own function; fixed no mail on missing key issue and added test

Task #11536: Changing stopping criterium from sum(|old-new|^2)/sum(|old|) to sum(|old-new|^2)/sum(|old|^2)

Task #11536: Adding keyword 'ddecal.statfilename', which can be used to save a simple txt file with the convergence information

Task #11536: Removing dependency on Armadillo, replacing linear solver by direct calls to (open)Blas