Show "# doctest: +SKIP" lines in the output.
Avoid copying the wcslib header files from cextern unnecessarily on each run of setup.py build.  This will only copy them if they are missing/out of date.
Merge pull request #2551 from mdboom/docs/show-doctest-literals

# doctest: +SKIP  needs to be escaped in documentation examples
Merge pull request #2569 from embray/build/unnecessary-copy

Skip unnecessary copy of wcslib headers on each build.
With quantity output, ensure non-quantity input is treated as dimensionless