[fix] make modStrings open and saveable (NOOPs), make a stringset over a modString be assignable a stringset over its host's type
[NOP] cleanup f7e4afc17dc931fda3ade404da4d79aba4e14de5
[fix] comments 2 and 3
[fix] remove initStringSetLimits where not needed
Merge pull request #625 from h-2/fix/StringSet<ModifiedString>

Fix/string set<modified string>
Merge branch 'develop' of github.com:seqan/seqan into feature/stream-fixes