core/kmeraffect.{h,cpp}: remove KmerStringAffect

KmerStringAffect is no more used in the code.
If one day we would like to "store more things" in the affectations,
we could still extend KmerAffect with a pointer to something else.
core/affectanalyser.{h,cpp}: De-templatization of KmerAffectAnalyser

As KmerAffectAnalyser is only used with KmerAffect, we do not need to keep the templates.
The code should be easier to read for C++ newbies such as the author of this commit.
tests: update unit tests to the de-templatized KmerAffectAnalyser