Add a method to change the data storage base path for on-disk backends

[jorainer]

On-disk backends depend on the (original) data files being available. Moving/copying serialized Spectra objects with such backends between file systems or moving the original data files will break their functionality.

This PR adds a new method to the MsBackend, dataStorageBasePath() and dataStorageBasePath<- to get or set the base path to the directory of the data storage files for on-disk backends (issue #321 ).