Add iterative wcs.all_world2pix

Iterative inverse of wcs.all_pix2world. Closes #1066.
Add tolerance option for wcs.all_world2pix
Add unit test for wcs.all_world2pix
Rename pixes to pix

The name of this variable is meant to echo the name of the function,
world2pix.
Skip test_all_world2pix if scipy is not installed
Separate methods with newline
Merge pull request #1281 from lpsinger/iterative_all_world2pix

Add iterative wcs.all_world2pix