geoviews.operation.regrid.weighted_regrid() does not respect file_pattern parameter

[Timothy-W-Hilton]

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ALL software version info (Panel, bokeh, python, notebook, OS, browser, any other relevant packages)

geoviews 1.6.5 py_0 conda-forge
geoviews-core 1.6.5 py_0 conda-forge
holoviews 1.12.6 py_0 conda-forge
bokeh 1.3.4 py37_0 conda-forge
python 3.7.3 h93065d6_1 conda-forge
notebook 6.0.1 py37_0 conda-forge
MacOS High Sierra 10.13.6
Safari Version 13.0.4 (13608.4.9.1.4)

Description of expected behavior and the observed behavior

I expected supplying a file_pattern string and reuse_weights=True to geoviews.operation.regrid.weighted_regrid() to cause weighted_regrid() to write a weight file with the name supplied, and to reuse that file on subsequent regrids. It seems the file_pattern value is overwritten with the default somewhere in Operation.call() in operation.py (beginning line 168 in my installation)

Complete, minimal, self-contained example code that reproduces the issue

Placing these two lines after the final xEMSF example in the notebook at http://geoviews.org/user_guide/Resampling_Grids.html reproduces the behavior. Both calls produce the output "Create weight file: bilinear_(0.356, 449.650)_(16.334, 89.638)_400x400.nc". I've posted a complete example .

weighted_regrid(quadmeshes, streams=[], reuse_weights=True, file_pattern=('weights_1.nc')) * gv.feature.coastline
weighted_regrid(quadmeshes, streams=[], reuse_weights=True, file_pattern=('weights_2.nc')) * gv.feature.coastline

Stack traceback and/or browser JavaScript console output

Screenshots or screencasts of the bug in action

[ppwadhwa]

This bug should probably be fixed along with #406