Standardise use of HDF5Proxy #270
Description
In the lazy expression machinery and metadata recovery, whenever a new call is mae (to create a lazyexpr or read metadata), one calls HDF5Proxy(path) on the unfolded .b2nd path. It would be good to avoid this to make the code cleaner I think, and somehow have the HDF5 proxy metadata made available upon unfolding and not create a new instance every time.