GMOS MDF Cut

This completes the Prepare task: it mosaics and cuts now the little slits. There's also a lot of information on the wavelength solution all calculated from the grating information. 

![flat_slits](https://f.cloud.github.com/assets/506116/456861/9cea65ec-b37a-11e2-8b8e-54fa2aba7bff.png)

These are all 48 slits of a flat field - one can nicely see the chip gap that is masked. 

There's still a small problem of the translation between fits and numpy (beginning with 1 or 0). Something to figure out for later.