Enabling slicing of PooledBuffers?

[zachjweiner]

Re: geggo/gpyfft#32

Right now, it seems PooledBuffers can't be sliced like normal buffers, which means gpyfft can't turn PooledBuffers with offsets into new buffers without them (as required for clfft). I tried blindly changing, e.g., parent classes and copying methods into the wrapper for PooledBuffer, but quickly got out of my depth with build errors. Would it be possible to enable this?

@geggo has more insight into the issue than I, see geggo/gpyfft#32 (comment).