Work toward using class for running fmask
Updated unloading of plugin to delete results (good for plugin reload)
Added ability to cache TOA/BT retrieval for plcloud

plcloud normally runs 'nd2toarbt' which reads in from TIF DN images the
Landsat data, converts it, and then proceeds. If we're running many
iterations of plcloud, then this is unnecessary (and expensive - ~30s for
nd2toarbt vs ~10s for rest of plcloud). Disabled by default, this checkbox
option will allow memory storage of the top of atmosphere reflectance and
brightness temperature (disabled by default for memory concerns)