Change default setting for `group_by` in the DICOM reader

[sluijs]

The group_by argument of DicomReader currently defaults to EchoNumbers, which is not commonly present in DICOM files.

Typically, I would only load one DICOM study and not use grouping at all. I would suggest to default to None and to disable grouping that scenario (i.e., not returning a list of MedicalVolume, but a single instance).

[EAlexWaters]

May I suggest instead having the group_by argument default to EchoTime?

This accomplishes:

• Makes DOSMA not crash when you try to load DICOMS that don't implement EchoNumbers
• It's a mandatory tag, so all MRI DICOM files should have it
• Handles the case where you only have one echo time and don't need grouping