switched boxm2_cache2 and boxm2_opencl_cache2 to defaults

The old boxm2_cache and boxm2_opencl_cache have been preserved as
boxm2_cache1 and boxm2_opencl_cache1.  What was previously called
boxm2_cache2 and boxm2_opencl_cache2 have been renamed to boxm2_cache
and boxm2_opencl_cache.  The new accessors take a boxm2_scene_sptr
argument in addition to the block id, so all code accessing data via
boxm2_opencl_cache interface has been updated to pass the additional
arguments.
test centroid of block in addition to corners
more refactoring and bugfixes
pass scene into merge_block_function
bugfix: change delete[] to delete
add exected_image_renderer class

This class will eventually do the heavy lifting for the expected image
rendering process.  Most of the code is a copied directly from there.
remove unused input
make block retrieval work for named apm models
allow named appearance models
use new cache type, allow non-perspective cameras
fix comment
move heavy lifting out of processes, into classes
Merge branch 'switch_to_opencl_cache2'

Conflicts:
	contrib/brl/bseg/boxm2/boxm2_scene.cxx
	contrib/brl/bseg/boxm2/pyscripts/boxm2_adaptor.py
fix merge-related errors