ENH: Scale image with DQ mask (convolution)

[pllim]

Fix #199 . To be used by spacetelescope/wss_tools#70 .

TODO

• [x] Write test.
• [x] User acceptance.
• [x] Squash commits.
• [x] Merge PR.
• [x] Open issue about rescale_wcs. See #205
• ~Release stginga.~ Will ask later.

[pllim]

Adam Ginsburg said try convolve_fft and see if it is any faster.

[pllim]

As discussed in https://github.com/spacetelescope/stginga/pull/201#issuecomment-708419740 , this is the winner.