Added control flow to bfmSpatial.

First, if a value for sensor is given, names(x) must correspond to Landsat sceneID's, otherwise this will be ignored with a warning message.

Second, if all values in a pixel stack are NA, it doesnt make sense to run bfastmonitor (and previously, this would return an error, and therefore a value of 1 to the error layer). Now, all output pixel values are NA if the original values for a pixel stack were all NA's (ie. if a mask had been previously applied).
Merge branch 'feature-bfmSpatial' into develop
merge previous commit

Merge branch 'develop' of http://github.com/dutri001/bfastSpatial into develop

Conflicts:
	R/bfmSpatial.R
resolve conflicts