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
Announce addition of `all_world2pix()` method

See #1281.
Merge pull request #1304 from lpsinger/iterative_wcs_whatsnew

Announce addition of `all_world2pix()` method
Fix for io.votable.validator
Record new method `WCS.all_world2pix` in changelog
Merge pull request #1307 from lpsinger/iterative_wcs_whatsnew

Record new method `WCS.all_world2pix` in changelog
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Branch rebase and related changes