re: PR https://github.com/Unidata/netcdf-c/pull/2716).
re: Issue https://github.com/Unidata/netcdf-c/issues/2189
The basic change is to make use of the fact that HDF5 automatically suppresses optional filters when an attempt is made to apply them to variable-length typed arrays.
This means that e.g. ncdump or nccopy will properly see meaningful data.
Note that if a filter is defined as HDF5 mandatory, then the corresponding variable will be suppressed and will be invisible to ncdump and nccopy.
This functionality is also propagated to NCZarr.
This PR makes some minor changes to PR https://github.com/Unidata/netcdf-c/pull/2716 as follows:
* Move the test for filter X variable-length from dfilter.c down into the dispatch table functions.
* Make all filters for HDF5 optional rather than mandatory so that the built-in HDF5 test for filter X variable-length will be invoked.
The test case for this was expanded to verify that the filters are defined, but suppressed.
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