Parse spectra and chromatograms from mzml mzxml via cpp

[ricardo-cunha]

• xml_utils.cpp (xml_utils.h) holds the cpp functions
• rcpp_parse_(...) are the R interface functions via rcpp
• external_libraries.hpp is used to include xmlpugi but without PUGIXML_NO_XPATH as it is used. This could be changed by adapting the PATROON_PARSE_XML_H I think.
• Makevars and Makevars.win were changed to include the zlib library

[rickhelmus]

General TODO

• [ ] style consistency: snake_case to camelCase
• [ ] style consistency: start braces on next line
• [ ] style consistency: indent with four spaces
• [ ] rename xml_ files to something less generic
• [ ] include pugixml header consistently and from one place
• [ ] add in the right references (mz(X)ML loading, base64 etc)
• [ ] move all files to subdirectory?