Calculate Euler numbers and place them in the sidecar json of surfaces.

[oesteban]

This issue entails:

• Sending a PR to nipype with an interface for mris_euler_number (we could have it first in niworkflows if that eases the process).
• Adding a node to smriprep

Ref: http://freesurfer.net/fswiki/mris_euler_number

[mgxd]

@oesteban what did you mean by

sidecar json of surfaces

[oesteban]

That we could write out the euler number as part of the json corresponding to the surf.gii outputs.