Description:
Previously, there was no versioning for H5Z_class_t. This prevented applications
written for 1.6 using custom filters from being able to use the 1.8 library.
There is now an H5Z_class1_t and H5Z_class2_t to enable compatibility. H5Zregister is
*not* versioned, it determines which version of the struct has been passed in by the
value of the first field (id or version, both are ints).
Tested: jam, linew, smirom (h5committest), jam (--with-default-api-version=v16)
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