Peaks and spectra data accessors and replacement methods

[lgatto]

Discussion stemming from the package review:

• keep as(Spectra, "SimpleList") and as(Spectra, "list") - these would call peaksData,MsBackend. The setAs method would be the preferred way to access the peaks as a list.
• keep spectraData() and drop all asDataFrame, as(...) ... because we can't use additional arguments (spectraVariables = ...), for Spectra and MsBackend
• also have a peaksData that does the same as as(Spectra, "SimpleList") - and also a method for the backends. This method is added so that we can have a clean name and syntax for the replacement method (see below) And for the replacement methods:
• spectraData<-,MsBackend
• peaksData<-,MsBackend

Related issue: #86

[jorainer]

All has been implemented now and was pushed also to BioC. Now lets wait for Marcel's reply.