Add LC-MS feature grouping functionality to xcms

[jorainer]

• Add LC-MS feature grouping functionality based on the MsFeatures package to xcms.
• Add a vignette describing this grouping.
• Adapt to recent changes in ProtGenerics and MSnbase (some methods were moved from MSnbase to ProtGenerics).

[jorainer]

Note that the unit tests fail on macOS and Windows because of the warning of mzR being built with a different Rcpp version.

[jorainer]

Thanks @sneumann for your comments! I made now some changes (e.g. add parameter transform to plotChromatogramsOverlay to allow data transformation).

Regarding the naming: I totally agree. I would keep the names of the functions as they are (i.e. groupFeatures) because they are defined in MsFeatures and actually are just defining groups - anything like combine aggregate would imply IMHO already a combination of the actual values. If OK for you I would do some rewording in the documentation and the vignette to replace feature grouping with e.g. the compounding that you suggested (is that in fact a recognized word in untargeted LC-MS analysis?).

[jorainer]

@sneumann , I've reformulated part of the documentation and vignette also introducing the term compounding. Please have a look at it if you find the time.

[sneumann]

looks good to me now.