spectraData,Spectra does not return m/z and intensity values

[jorainer]

This PR (discussed in #92)

• replaces the spectraData,MsBackend method with asDataFrame,MsBackend (and spectraData<-,MsBackend with asDataFrame<-,MsBackend.
• changes spectraVariables,Spectra to not return "mz" and "intensity" with the result that spectraData,Spectra does return only a DataFrame with spectra metadata, no m/z and intensity values.
• add a method dropNaSpectraVariables to allow application of this function to both Spectra and MsBackend (the function that was in place before used spectraData, that now however does no longer exist for MsBackend. I thought this function might still be useful for MsBackend and that's why I added it as a method.
• Updates the documentation and fixes the unit tests.

[jorainer]

@sgibb, should I wait for your review or am I OK to merge?

[codecov-io]

Codecov Report

Merging #95 into master will decrease coverage by <.01%. The diff coverage is 95.31%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
- Coverage   90.17%   90.17%   -0.01%     
==========================================
  Files          16       16              
  Lines        1456     1455       -1     
==========================================
- Hits         1313     1312       -1     
  Misses        143      143

Impacted Files	Coverage Δ
R/Spectra-functions.R	96.45% <ø> (-0.13%)	:arrow_down:
R/MsBackendMzR.R	97.05% <ø> (ø)	:arrow_up:
R/Spectra.R	93.05% <100%> (-0.13%)	:arrow_down:
R/MsBackend.R	10.14% <100%> (+7.01%)	:arrow_up:
R/AllGenerics.R	100% <100%> (ø)	:arrow_up:
R/MsBackendDataFrame.R	97.86% <86.66%> (ø)	:arrow_up:
R/MsBackendHdf5Peaks.R	90.28% <96.15%> (ø)	:arrow_up:

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