Added rsun_obs property to return a quantity

[MSKirk]

Description

Added rsun_obs property to stereo map sources. It returns a quantity in arcsec consistent with other Maps and overwrites mapbase's assumption of a photospheric limb as seen from Earth.

[pep8speaks]

Hello @MSKirk! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file sunpy/map/sources/tests/test_euvi_source.py:

Line 37:1: E302 expected 2 blank lines, found 1
Line 41:1: E302 expected 2 blank lines, found 1
Line 45:101: E501 line too long (107 > 100 characters)

Comment last updated at 2019-05-14 14:21:41 UTC

[ghost]

Thanks for the pull request @MSKirk! Everything looks great!

[nabobalis]

Any chance of a test and a changelog entry?

[codecov]

Codecov Report

Merging #3099 into master will decrease coverage by 0.1%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #3099      +/-   ##
==========================================
- Coverage   89.96%   89.86%   -0.11%     
==========================================
  Files         148      148              
  Lines       10557    10564       +7     
==========================================
- Hits         9498     9493       -5     
- Misses       1059     1071      +12

Impacted Files	Coverage Δ
sunpy/map/sources/stereo.py	83.01% <28.57%> (-8.29%)	⬇️
sunpy/map/map_factory.py	91.6% <0%> (-1.4%)	⬇️
sunpy/net/helioviewer.py	93.25% <0%> (-1.13%)	⬇️
sunpy/instr/fermi.py	88.57% <0%> (-0.72%)	⬇️
sunpy/net/attr.py	88.54% <0%> (-0.45%)	⬇️
sunpy/database/tables.py	92.15% <0%> (-0.35%)	⬇️
sunpy/database/database.py	95.75% <0%> (-0.29%)	⬇️

---

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[MSKirk]

Done.

[nabobalis]

Thanks!

[wafels]

This has been missing for a long time. Ugh. Thanks for fixing this @MSKirk.

As a general comment, it might be worth someone's time to go through all our other map sources to find out which properties fall back on defaults, and then to find out if this is justified.

[nabobalis]

Conda build failure is unrelated to the PR.