more control for 3d flag in roms_dataset

[kthyng]

change to roms_dataset() so that input flag include_3D_metrics also controls if ds["3d"] = True.

[codecov-commenter]

Codecov Report

Merging #59 (b2fed17) into main (021d7ed) will not change coverage. The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   83.93%   83.93%           
=======================================
  Files          17       17           
  Lines        1898     1898           
=======================================
  Hits         1593     1593           
  Misses        305      305           

Flag	Coverage Δ
unittests	83.93% <100.00%> (ø)

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Files	Coverage Δ
xroms/xroms.py	58.42% <100.00%> (ø)