Fix ancillary variable loading when anc var is already loaded

[djhoese]

In a previous PR I broke ancillary variable loading if the anc var is loaded multiple times. This PR fixes that as well as makes it "illegal" to request a key from a DatasetDict that isn't a str, number, or DatasetID.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage increased (+0.002%) to 69.129% when pulling e652d18 on djhoese:bugfix-anc-vars into a1ce978 on pytroll:master.

[codecov]

Codecov Report

Merging #341 into master will increase coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   69.12%   69.12%   +<.01%     
==========================================
  Files         123      123              
  Lines       15632    15636       +4     
==========================================
+ Hits        10806    10809       +3     
- Misses       4826     4827       +1

Impacted Files	Coverage Δ
satpy/readers/yaml_reader.py	85.31% <100%> (+0.06%)	⬆️
satpy/readers/__init__.py	94.81% <50%> (-0.34%)	⬇️

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