Hdf5 io operators should use read_direct and write_direct when possible.
BlockwiseFileset:
- Use read_direct and write_direct.
- Added exportRoiToHdf5()
BlockwiseFileset: Allow compression.
BlockwiseFileset: Clip chunkshape to dataset shape if necessary.
BlockwiseFileset: Add __enter__ and __exit__ methods for context manager support.
BlockwiseFileset: Added a factory for creating blockwise fileset (sub)class instances from a description file.
OpCompressedCache: setInSlot() can be used for arbitrary rois (but see comment about rammifications for dirty blocks)