PixelOperator may serve as a handy base class for an operator that computes any number of target samples
from any number of source samples.
PointOperator class serves as a base class for operators
that compute single pixels independently of their neighbours and
whose target product and all source products share the same grid and coordinate reference system.
SampleOperator may serve as a handy base class for an operator that computes a single target sample from
any number of source samples.