Add possibility to change the file path for a `MsBackendMzR`

[jorainer]

We have some use cases in which we run some part of the analysis (e.g. the preprocessing using xcms) on our cluster, save then the results (which include a Spectra) to an RData to then continue the analysis (more interactively) on a local computer. This works only if we use a MsBackendMemory as a backend, but e.g. with a MsBackendMzR we would have different file paths locally and on the cluster.

Maybe we could add a method backendFilePath,MsBackend and backendFilePath<-,MsBackend that allows to get/change the directory where the data is stored - for backends that support this.

A method could be implemented for MsBackendMzR, MsBackendHdf5Peaks and eventually the MsBackendOfflineSql, if a SQLite database is used. Other backends would throw an error as this method would not be supported/needed for them.

Open for discussions or alternative ideas/thoughts.