hdf5_msb

Modifications for the following items in the punch list:
(A) #5: Add the "pb_expansion_threshold" field to the "H5F_vfd_swmr_config_t" structure
and update H5Pset_vfd_swmr_config() and H5Pget_vfd_swmr_config() accordingly
(B) #13 bullet 2: Comment H5F_vfd_swmr_config_t in H5Fpublic.h properly
(copied from John's description in the RFC)
(C) Change the field name "vfd_swmr_writer" to "writer" in "struct H5F_vfd_swmr_config_t"
(as indicated on page 11 in the RFC) and all references to it