Work-around for special modifications in Mascot (fixes #864)

Some modifications are only supported as specificity groups by Mascot (e.g. "Deamidated (NQ)", "Phospho (ST)"), but not by OpenMS. I've added a list of these modifications (taken from Mascot 2.4) to the code and treated them as special cases, in which the OpenMS notation needs to be replaced by the Mascot notation when an MGF file is written.
Tested and found to work with Mascot 2.4.
you need to move this code to `void updateMembers_();` (which is currently not overloaded) otherwise the parameter value will not be transferred if a user calls `MascotGenericFile::setParameters()`