[FIX] enable loading of pepXML files that contain analysis summaries

Some files could not be loaded before due to a segmentation fault.
The problem was that an element named "search_summary" can occur within "analysis_summary" and within "msms_run_summary" (where we parse it), but with different meanings.
Now the complete "analysis_summary" element is skipped to avoid this problem.
[FIX] enable loading of pepXML files where searches of the same file are numbered consecutively across 'msms_run_summary' sections ('search_id' attribute in 'search_summary')
[COMMENT] added explanation for code dealing with numbering of searches
Merge branch 'develop' of https://github.com/OpenMS/OpenMS into fix-pepxml-load

Conflicts:
	src/openms/source/FORMAT/PepXMLFile.cpp
Merge branch 'develop' of https://github.com/OpenMS/OpenMS into fix-pepxml-load
Merge pull request #892 from hendrikweisser/fix-pepxml-load

[FIX] Enable loading of some pepXML files that used to cause crashes