do not copy results back from OpenCL by default

The new TransformDomainSignal representation for the output class has
allowed one to abstract the copying from device to host until the last
possible moment by introducing a compatible TransformDomainSignalOpenCl
class. The copy is performed at attribute access time. It may be
desirable to memoize the output which will be implemented in a separate
commit.
move memoize decorator to dtcwt.utils

The decorator is about to be used elsewhere. It's worth moving to a
standard place.
memoize any to-host copies in OpenCL transform result
Merge pull request #34 from rjw57/opencl-no-host-copy

add support for non-copy-back OpenCL transforms