nonspecific search engine bug fix and unit test

[trishorts]

non-specific search engine has a bug where a second modification is attempted to be added to a peptide with set mods where there is an existing modification at that position. This PR adds a check that prevents addition of the second mod.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.87%. Comparing base (a06bc0d) to head (94c7354).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2453      +/-   ##
==========================================
+ Coverage   93.82%   93.87%   +0.04%     
==========================================
  Files         142      142              
  Lines       22008    22008              
  Branches     3014     3014              
==========================================
+ Hits        20648    20659      +11     
+ Misses        910      903       -7     
+ Partials      450      446       -4     

Files with missing lines	Coverage Δ
...cificEnzymeSearch/NonSpecificEnzymeSearchEngine.cs	88.95% <100.00%> (+1.47%)	⬆️

... and 1 file with indirect coverage changes