Give error if running unsupported version of 6S

[robintw]

Fixes #31.

[codecov]

Codecov Report

Merging #87 (d64fdfc) into master (173501a) will decrease coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   87.75%   87.71%   -0.05%     
==========================================
  Files          18       18              
  Lines        1715     1717       +2     
==========================================
+ Hits         1505     1506       +1     
- Misses        210      211       +1     

Impacted Files	Coverage Δ
Py6S/sixs.py	95.00% <50.00%> (-0.57%)	⬇️

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