Fix issues with really large label filesize

[gdevenyi]

Either figure out how to make ITK output compressed minc files, or add in optional MINC tools usage

See: http://stackoverflow.com/questions/592620/check-if-a-program-exists-from-a-bash-script

[gdevenyi]

Don't forget about checking for the extension of the label file.

[gdevenyi]

Ties in with #21 since it's easier to do checks inside another script.

[gdevenyi]

In progress via https://github.com/stnava/ANTs/pull/305

[gdevenyi]

Fixed