fix memory leak in OpenCL implementation

The lowpass, subbands and scales properties of
backend_opencl.TransformDomainSignal were indeed memoized but memoized
globally meaning that referencing them would keep a reference to the
TransformDomainSignal object indefinitely leading to memory growing
without bound. Replace the @memoize decorator with a simple
attribute-based cache.
update ChangeLog for 0.7.1
release 0.7.1
add changelog for 0.7.2
release 0.7.2