fix: FutureWarning in coslat check

[slevang]

One more small one. When supplying a custom array to weights in the xarray interface, we get:

FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  use_coslat = True if weights == 'coslat' else False

This just checks if weights is a str before trying to compare so we don't get the warning.

[codecov-commenter]

Codecov Report

Merging #37 (d798cb6) into main (1e1694f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   96.28%   96.28%           
=======================================
  Files          49       49           
  Lines        3206     3206           
=======================================
  Hits         3087     3087           
  Misses        119      119           

Impacted Files	Coverage Δ
xeofs/xarray/eof.py	100.00% <100.00%> (ø)
xeofs/xarray/mca.py	98.05% <100.00%> (ø)
xeofs/xarray/rock_pca.py	98.11% <100.00%> (ø)

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[nicrie]

Thanks again Sam! I'll pull the PR as soon as the conda-forge review for version 0.7.1 has finished!