Distort wcs

[esheldon]

closes #48

TODO

• add code in the sim to use this wcs
• add more than one example wcs
• keep the header around so we can use it to create the wcs in the lsst code
• make descwl_coadds look for that header and use if it it is available

[esheldon]

the galsim wcs had the ability to write out the fits hdr keywords to a dict, so we don't need to pass along the header

[esheldon]

Use the wcs._writeHeader method.

[beckermr]

We might want to ask @rmjarvis if we can convert this to a public API so it doesn't change on us.

[esheldon]

There is a public one writeToFitsHeader but it takes a bounds instance that is not needed

[beckermr]

A writeToDict method might be useful generally and fit the bill.

[esheldon]

looking at the code, I guess I could fake it with galsim.BoundsI(1, anything, 1, anything) since it only checks that the xmin and xmax are 1

[esheldon]

that code will go into descwl_coadd, so I'm ready to merge this one if you are matt

[esheldon]

I ran tests locally, true they aren't running in travis since we don't have the catsim data

[beckermr]

But the tan-sip doesn't use catsim, so why have the decorators there at all?

[esheldon]

It reads data from CATSIM_DIR