Spatial bfast output / relation to bfastSpatial

For ease of applicability, having `bfast` generate spatial maps rather than only pixel time series would be good to have. `bfastSpatial` has some of that functionality, specifically with `bfmSpatial`, but it only deals with `bfastMonitor` output. Also, `bfastSpatial` is useful as a generic preprocessing framework, the output does not necessarily have to be used with `bfast`.

So an idea would be to add something like `bfSpatial` for the core `bfast` function. Perhaps with coordination with Loïc.

No files or tests are named. Start by comparing the core bfast function with bfastSpatial's bfmSpatial output, then clarify the intended spatial-map interface and coordination with Loïc. Done requires an agreed scope and implementation plan for spatial output from bfast.

Written by the indexing model from the issue text.