Don't subtract CRPIX from input values for the inverse SIP transformatio...

...n

According to the SIP paper the inverse transformation is applied directly to the input values, and not relative to `CRPIX`. 

I have difficulties interpreting the output of the inverse transformation for the sample data provided here

http://fits.gsfc.nasa.gov/registry/sip.html

so I can't come up with a good test for this but I think it's clear from the paper that `CRPIX` should not be subtracted from the input values.

@mdboom I am opening the issue for consideration. Feel free to implement it in a different way.