Split MRS metadata table to match that of MRI

[Remi-Gau]

https://bids-specification.readthedocs.io/en/latest/modality-specific-files/magnetic-resonance-spectroscopy.html#scanner-hardware

should be split by:

• hardware information
• institution information
• timing information
• RF and contrasts
• Slice acceleration
• ...

this is just some refactoring so this should not be required to the MRS release