Clean-up of docstrings and log messages related to meta

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Description of PR

This PR cleans up a few inconsistencies in the documentation of the meta indicators and changes the recommended arg of as_pandas() to meta_cols (deprecating with_metadata).

[danielhuppmann]

@gidden, this PR refactors (among other clean-up operations) the arg of as_pandas() from with_metadata to with_meta for consistency - but looking at the function and its uses, I'm wondering if it wouldn't make more sense to change the behavior:

• return data merged with all meta columns by default
• return data merged with specific meta columns (by passing a list, used by the plotting functions)

If a user just wants data (current default), she/he can just use df.data.copy() directly, no need to use this wrapper... Any objections?

[codecov]

Codecov Report

Merging #402 into master will increase coverage by 0.08%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
+ Coverage   93.42%   93.50%   +0.08%     
==========================================
  Files          35       35              
  Lines        4046     4067      +21     
==========================================
+ Hits         3780     3803      +23     
+ Misses        266      264       -2     

Impacted Files	Coverage Δ
pyam/core.py	92.36% <77.27%> (-0.02%)	⬇️
pyam/plotting.py	95.47% <100.00%> (+0.07%)	⬆️
tests/test_admin.py	95.00% <100.00%> (ø)
tests/test_core.py	99.66% <100.00%> (+<0.01%)	⬆️
pyam/logging.py	100.00% <0.00%> (+16.66%)	⬆️

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[danielhuppmann]

As there were no objections against the changed behavior of as_pandas() - I'm going to merge this PR tomorrow...