Allow users to provide their own seed image+segmap into various modes

[bhilbert4]

Inspiration for this is a help desk ticket where the user was trying to break up his very large WFSS simulation into multiple pieces to save memory. Would be nice to be able to generate the full seed image, then break it up manually, and then feed those pieces back into Mirage to continue on.

This should already be possible for imaging mode, even if it's not well documented.