- Add multithreading capabilities to h5mread().

- Make h5mread(..., as.sparse=TRUE) natively return an SVT_SparseArray rather
  than a COO_SparseArray object.

- h5mread():
    * Support datasets with zero dimensions i.e. with ndim (a.k.a. rank
      in HDF5 terminology) = 0.
      system.file("extdata", "krumsiek11.h5ad", package="zellkonverter")
      contains several of such datasets.
    * Support compound datasets.

- Resume work on h5summarize().

- The reshaping operation that is currently performed by
  h5mread_from_reshaped() should instead be implemented as a
  delayed operation on DelayedArray objects.

