2. Factored out the parameter string parsing for ncgen and nccopy
int libdispatch/dfilter.c + include/ncfilter.h
3. Allow a parameter string to use constant types other than
unsigned int. See docs/filters.md for details.
4. Moved the old content of include/netcdf_filter.h into include/netcdf.h
and removed include/netcdf_filter.h as no longer needed.
5. Force the test filter (bzip2) in nc_test4/filter_test to
be built using BUILT_SOURCES.
