New attribute: "is map empty?"

[cmaumet]

As discussed with @vsoch and @chrisfilo at vsoch/nidmviewer#8, it would be useful to include an attribute for each NIfTI image (nidm:'Map') to indicate whether or not the map is empty.

I think that the sha-shum is not sufficient because of the possible presence of NaNs to define the image background.

[vsoch]

This is essential I think for nidm - needing to load the map via a url to check if it's empty is not always a reasonable thing to do, depending on the server / application, etc. See NeuroVault/NeuroVault#521 (comment)