enhance dataset_description and sidecars to please the new bids_validator

[bpinsard]

Summary

Running the new bids-validator I get more warnings than with the older one.
eg

	[WARNING] JSON_KEY_RECOMMENDED A JSON file is missing a key listed as recommended.
		DatasetType
		/dataset_description.json

		GeneratedBy
		/dataset_description.json

		SourceDatasets
		/dataset_description.json

So we should maybe improve the currently generated dataset_description.json to include such tags.

It also get errors such as

	[ERROR] SIDECAR_KEY_REQUIRED A data file's JSON sidecar is missing a key listed as required.
		Units
		/sub-participant/ses-43/anat/sub-participant_ses-43_acq-stagefa5_echo-1_part-phase_MEGRE.nii.gz
		/sub-participant/ses-43/anat/sub-participant_ses-43_acq-stagefa5_echo-2_part-phase_MEGRE.nii.gz

So it seems that all part-phase should have Units defined in the json, in most cases this would be arbitrary.
As heudiconv can set part-phase after dcm2niix in update_complex_name, it should be done in heudiconv.

Platform details:

Choose one:

• Local environment

• Container

• Heudiconv version:

1.3.2