fix j dim datatype for llc grids

[rabernat]

This makes sure we have integer indices for llc grids.

[codecov-io]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files           4        4           
  Lines         608      608           
  Branches      134      134           
=======================================
  Hits          559      559           
  Misses         32       32           
  Partials       17       17

Impacted Files	Coverage Δ
xmitgcm/mds_store.py	92.02% <100%> (ø)	⬆️

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