Purpose: Implement H5Dscatter and H5Dgather
Description:
Adds 2 new API functions, H5Dscatter and H5Dgather. H5Dscatter retrieves data
from a specified callback function and scatters it into a selection, defined by
a supplied dataspace, within a supplied memory buffer. H5Dgather gathers data
from a selection within a supplied memory buffer and passes it in a contiguous
form to a supplied callback function. Added tests for these functions
Tested: jam, ostrich, koala (h5committest); ummon; ummon (1.8)
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