IMarkedIons Does not Support High Resolution MS

[kerner1000]

Ions are represented as int, better double or IIon should be used.

[laeubi]

IMarkedIons is only a container for IMarkedIon that support double already.

But the IMarkedIons is currently explicitly designed to only contain "integer ions". Even though I think it is possible to extend the idea of marked Ions to higher precision (e.g. specifying a desired number of digits to match, replacing getIonsNominal() with a "matches" method) we should first describe what is the use-case for this, for example why/when/where/how would a user like to describe ions in a higher resolution.