Allow expanding single keys to multiple keys.

[dcherian]

.isel(X=5, Y=5) → .isel(xi_rho=5, xi_u=5, xi_v=5, eta_rho=10, ...)

Closes #13

[codecov-commenter]

Codecov Report

Merging #27 into master will increase coverage by 0.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   87.33%   87.92%   +0.58%     
==========================================
  Files           6        6              
  Lines         308      323      +15     
==========================================
+ Hits          269      284      +15     
  Misses         39       39              

Flag	Coverage Δ
#unittests	87.92% <100.00%> (+0.58%)	⬆️

Impacted Files	Coverage Δ
cf_xarray/accessor.py	90.53% <100.00%> (+0.05%)	⬆️
cf_xarray/tests/test_accessor.py	100.00% <100.00%> (ø)

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