ENH: Support applying a transform to MRSBase

[bennyrowland]

In cases where multiple spectroscopy exams are performed on the same subject, it is often necessary to register them to each other. This is usually done by registering co-acquired structural images together to get a transform between the patient position in each case, then that transform must also be applied to the spectroscopy data. Therefore it should be possible to apply a transform to an MRSBase object to update its internal transform to a new space. This will not affect the internal data, but will be relevant for resampling and superimposing on structural images.