Add __slots__ to classes that subclass xr.DataArray

[ttung]

The absence of __slots__ triggers a warning.

Test plan: travis alltest!

[shanaxel42]

thank you for doing this!

[codecov-io]

Codecov Report

Merging #1607 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1607      +/-   ##
==========================================
+ Coverage   88.04%   88.05%   +<.01%     
==========================================
  Files         151      151              
  Lines        5171     5175       +4     
==========================================
+ Hits         4553     4557       +4     
  Misses        618      618

Impacted Files	Coverage Δ
...sh/core/intensity_table/decoded_intensity_table.py	100% <100%> (ø)	⬆️
starfish/core/codebook/codebook.py	96.77% <100%> (+0.02%)	⬆️
starfish/core/intensity_table/intensity_table.py	96.29% <100%> (+0.03%)	⬆️
...arfish/core/expression_matrix/expression_matrix.py	52% <100%> (+2%)	⬆️

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[ttung]

@shanaxel42 said ok on slack. landdddiiiiiiing.