Rodent T2w preprocessing: calculate T2w template and validate headers

[oesteban]

Based on https://github.com/poldracklab/smriprep/blob/4f79b71e5527a1b4c6844e8508996386b26b75f0/smriprep/workflows/anatomical.py#L38-L381, set up a workflow for rodent MRI that:

• Calculates a T2w template if several images are given

[oesteban]

It seems we'll carry on with WHS. Closing this.