BUG: some Siemens spectroscopy DICOM files are not loaded

[bennyrowland]

Siemens DICOM files from VB17A (from Magnetom 7T) have a ProtocolSliceNumber parameters in the CSA header which is not handled by the load_siemens_dicom() function, leading to an error in import.

In addition, at this point there is probably no need to produce an error on an unrecognised CSA tag, as all essential tags are already being read in, all others are metadata at best and so if some are missing it shouldn't matter. Instead a warning could be raised which would allow users to report back if they find a new parameter to be missing, without it affecting their use of the software.