spacetelescope/drizzle: Added propagation of errors and DQ flags. Bug fixes.

What's Changed Improvements: Add support for co-adding with squared weights (can be used for standard error propagation) by @mcara in https://github.com/spacetelescope/drizzle/pull/163 Add support for propagation of DQ flags by @mcara in https://github.com/spacetelescope/drizzle/pull/206 Add a warning about ignoring pixfrac with Lanczos kernel by @mcara in https://github.com/spacetelescope/drizzle/pull/202 Bug Fixes: fillval and fillval2 should be independent by @mcara in https://github.com/spacetelescope/drizzle/pull/200 Fix issues with Lanczos resampling by @mcara in https://github.com/spacetelescope/drizzle/pull/198 Replace 'scale' with two independent parameters: 'iscale' and 'kscale' by @mcara in https://github.com/spacetelescope/drizzle/pull/203 Miscellaneous: Add a warning about possible issues with sinc interpolation in tblot by @mcara in https://github.com/spacetelescope/drizzle/pull/213 Full Changelog: https://github.com/spacetelescope/drizzle/compare/2.1.1...2.2.0