Add results interface objects

[mwaskom]

The idea is to do something like

>>> template = lyman.SubjectTemplate("subj01")
>>> template.seg_fname
/home/user/study/proc/subj01/template.seg.nii.gz
>>> template.seg_img
<nibabel.nifti1.Nifti1Image at 0x000000000>

where the attributes are properties with caching of the image objects.

Makes much sense for the template data, anatomical data, etc. Could be implemented at other levels (i.e. preproc timeseries). Should it just be user-facing, or should lyman use it? Is there a way to make it depend on a formal spec so it doesn't get out of sync with the workflows?