For the AW_MEAN regridding method, report the width as 1 instead of NA.

[JohnHalleyGotway]

This very minor change in convention make the logic easier for printing useful warning messages to the user. This is similar to the nearest, upper-left, upper-right, lower-left, and lower-right types, where by convention we report 1 instead of NA.

Some DTC staff were confused about whether the regridding width applies to the AW_MEAN option and printing an informative warning message and updating the data/config/README file will help clarify.

[JohnHalleyGotway]

Did changes in my local feature_1186_aw_mean branch, tested, merged into develop, and pushed up to github.