add Makie plotting

[aplavin]

Plots AbstractSkyCoords as x=lon, y=lat.
Also, add (unexported) lonlat function – I think it's generally useful.

One issue with testing: loading python packages (like astropy) and Makie together leads to segfaults. Currently, commented out astropy tests, but not sure what's the best approach.

[cgarling]

If you isolate the astropy and makie tests with SafeTestsets.jl do the segfaults persist?

[aplavin]

I think the segfault is caused by two conflicting shared libraries loaded into the same process, it's not julia module related.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.90%. Comparing base (8e1c7e4) to head (db4e804).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/UnitfulExt.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage   94.48%   90.90%   -3.58%     
==========================================
  Files           5        6       +1     
  Lines         145      165      +20     
==========================================
+ Hits          137      150      +13     
- Misses          8       15       +7     

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[aplavin]

Thanks for the assistance and a quick merge! Would you also register this please?

[giordano]

That's easier to do when you also bump the version within the PR you're proposing, as someone else will have to do that anyway.

[aplavin]

I don't think I can bump it in General...
Make a PR bumping the version in Project.toml: #60. Not sure useful or not, as that PR is such a trivial change, but would be nice to get all the changes actually registered.