Re-evaluate automatic detection of peaks variables

[jorainer]

The MsBackendMemory supports additional, arbitrary peaks variables which are stored separately from spectra variables. The $<- method tries to automatically detect such peaks variables and assigns them. This should be changed to let the user specifically select or define whether the variable should be a peaks variable or not.

[jorainer]

To be consistent with all other backends it's better to disable the automatic detection/conversion of spectra variables to peaks variables in MsBackendMemory as that can result in unexpected behavior.