Weights are kept in memory instead of being written to disk.

[huard]

This sets factor=True and filename=None in the call to ESMF.Regrid, and then calls get_weight_dict to obtain a dictionary of weights in memory instead of writing the weights to disk.

The Regridder interface is changed to pass weights explicitly (weights replacing filename and reuse_weights=True).

Computed weights are saved to disk using the to_netcdf method.

Fixes #75

I haven't modified the docs or notebooks.
Feel free to dismiss if this is not the API you were looking for.

[huard]

@raphaeldussin Would you be comfortable reviewing this PR ?

[raphaeldussin]

@huard no problem! will review it by the end of the week

[raphaeldussin]

@huard see my minor comments. Once this is resolved, I'm happy with it. Tests are passing locally.

[codecov-commenter]

Codecov Report

Merging #91 into master will increase coverage by 0.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   96.55%   97.32%   +0.77%     
==========================================
  Files           6        6              
  Lines         319      337      +18     
==========================================
+ Hits          308      328      +20     
+ Misses         11        9       -2     

Impacted Files	Coverage Δ
xesmf/backend.py	96.51% <ø> (ø)
xesmf/frontend.py	96.73% <100.00%> (+1.15%)	⬆️
xesmf/smm.py	100.00% <100.00%> (ø)

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[huard]

I've updated the notebooks (mostly removing the clean_weight_file calls). I think this would be ready for a second review.

[rabernat]

If this PR is ready to merge, perhaps it can be moved to https://github.com/pangeo-data/xESMF? Unfortunately this has to be done manually.

[huard]

Merged in pangeo-data/xESMF#3