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 #163
• Add support for propagation of DQ flags by @mcara in #206
• Add a warning about ignoring pixfrac with Lanczos kernel by @mcara in #202

Bug Fixes:

• fillval and fillval2 should be independent by @mcara in #200
• Fix issues with Lanczos resampling by @mcara in #198
• Replace 'scale' with two independent parameters: 'iscale' and 'kscale' by @mcara in #203

Miscellaneous:

• Add a warning about possible issues with sinc interpolation in tblot by @mcara in #213

Full Changelog: 2.1.1...2.2.0