Issues with double modifications

[straussmaximilian]

Describe the bug
There can be issues when there would be double-modifications on the same amino acid.
This can be the case, e.g. when having a modifcation on an n-terminus of a peptide and a modification on the amino acid of this particular peptide. Allthough it should be modified twice, the way AlphaPept parses the modifications it will only be added once.

To Reproduce
Example:
e.g. mod<^ (mod on n-terminus) & modP (mod on P) on sample PEPTIDE should be modmodPEPTIDE but will be modPEPTIDE.

Additional context
For now there will not be a programmatic solution but a warning that one should use modifications with double-mass instead.