MNT: Add manifest files for use by `fsl_get_standard`

Add manifest.txt files to allow the OMM templates to be used by the fsl_get_standard tool (https://fsl.fmrib.ox.ac.uk/fsl/docs/#/utilities/dataset_clitools?id=fsl_get_standard).

The intent of fsl_get_standard is to allow users and scripts to dynamically select standard templates, instead of hard-coding paths to (e.g.) $FSLDIR/data/standard/MNI152_T1_2mm.nii.gz. Assuming that the OMM templates are installed into $FSLDIR/data/omm/Oxford-MM-1/, users/scripts can select the OMM templates by setting the FSL_STANDARD environment variable, e.g.:

$ fsl_get_standard
/usr/local/fsl/data/standard/MNI152_T1_2mm.nii.gz
$ export FSL_STANDARD=omm/Oxford-MM-1
$ fsl_get_standard
/usr/local/fsl/data/omm/Oxford-MM-1/OMM-1_T1_head.nii.gz

Or by using the --standard option, e.g.:

$ fsl_get_standard --standard omm/Oxford-MM-1
/usr/local/fsl/data/omm/Oxford-MM-1/OMM-1_T1_head.nii.gz