Add an assert that there are less than 2^16 unique strings in a column

[TomHodson]

There is no ODC codec to handle the case of there being more than 2^16 unique strings in a column so this just adds a guard assert. Also fixes the formatting.

[FussyDuck]

All committers have signed the CLA.

[simondsmart]

Note that this should never be hit in practice, as the number of rows is (by default) constrained to 10k.

[simondsmart]

can you also fix the weird indenting. Then merge.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.28%. Comparing base (5fe161e) to head (7cbe8d4).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #17      +/-   ##
===========================================
- Coverage    87.30%   87.28%   -0.02%     
===========================================
  Files          189      189              
  Lines        15020    14928      -92     
  Branches         0     1352    +1352     
===========================================
- Hits         13113    13030      -83     
+ Misses        1907     1898       -9     

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