Change Standardize BPIs to accept n input rasters

[scw]

Pat Iampietro (CSUMB) mentioned an issue with the old BTM tool that he hopes is fixed: the old tool expects two (and exactly two) input BPI rasters for the standardization step. In his use, they may only use one raster, or sometimes need more than 2, and making the tool handle a flexible number of input rasters would be preferable.

As a stop-gap, at least don't require the second raster, so the tool works for single raster usecases.

[deepseadawn]

Yes, great!