Enable loading meta from csv

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR enables reading a meta dataframe from a csv file (and adds a test for it).

closes #495

[codecov]

Codecov Report

Merging #496 (00d03f6) into master (76c33a5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
+ Coverage   92.96%   92.97%   +0.01%     
==========================================
  Files          41       41              
  Lines        4747     4755       +8     
==========================================
+ Hits         4413     4421       +8     
  Misses        334      334              

Impacted Files	Coverage Δ
pyam/core.py	92.32% <100.00%> (+<0.01%)	⬆️
tests/test_io.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 76c33a5...ed87e16. Read the comment docs.

[danielhuppmann]

pinging @coroa for a review

[coroa]

Looks good! I was not aware of the possibility to pass in an pd.ExcelFile, that explains the previous code change.