Enhancements to `bedrock_mask` functions

[trchudley]

For better communication, bedrock_mask_from_bedmachine and bedrock_mask_from_vector should probably be renamed bedrock_mask to stable_ground, which is what it is really.

Additionally, bedrock_mask_from_vector could have the option to invert the mask, depending on whether the vector file is outlining stable ground or ice (currently assumes stable ground).