update "cell_method" parameter name to "climatological_statistic"

[corviday]

The PCEX backend is changing the name of the cell_method parameter to cell_methods. This will need to be updated in a couple places in this repository, plus requirements.txt updated to the new version of the PCEX backend.

[corviday]

The name of the previous cell_method parameter is now the generally more accurate climatological_statistic.