Replace C++ implementation of where by an efficient version

drp_stella makes quite heavy use of where in C++. This is reasonable, but the current implementation is very poor; it uses a string comparison for every element.

This needs to be replaced, probably by an implementation that uses a functor (or lambda). The replacement needs to be confirmed to be efficient, which probably means checking the assembler.

At the same time, please add overloads to use native LSST image types as appropriate; we should not be calling getArray() anywhere in C++ or python code (outside unit tests).