Fix issue with empty MsBackendMemory and automatic peak variable detection in MsBackendMemory

This PR addresses issues #268 and #269:

268: fix support for empty Spectra and MsBackendMemory.
269: change the original way how peaks variables are handled in MsBackendMemory: instead of guessing if a spectra variable is in fact a peaks variable (based on the data type and number of elements), additional peaks variables must now be specified by the user using the peaksVariables parameter in backendInitialize. This ensures compliance of MsBackendMemory with all other backends but still provides the user the possibility to use the additional functionality from MsBackendMemory.

rformassspectrometry / Spectra