[query] Consider adding import/export support for HDF5

[patrick-schultz]

HDF5 could be a natural file format for matrix tables, esp. block partitioned and/or higher-dimensional generalizations. More near term, HDF5 is used for large single cell data, and adding import would allow hail to be used. https://hail.zulipchat.com/#narrow/stream/123010-Hail-Query-0.2E2-support/topic/Convert.20matrix.20in.20hdf5.20to.20hail.20MatrixTable/near/421945261