smurf: New command jsasplit

Splits a map or cube into JSA tiles. Currently, it assumes the input NDF
is gridded on the JSA all-sky pixel grid. But it should be changed so that
it will automatically resample the input NDF if it is not on the JSA
all-sky pixel grid.
smurf: Rename tile commands to start with "JSA"

This is so that all JSA-related smurf commands start with "JSA" to
indicate their purpose.