re: issue https://github.com/Unidata/netcdf-c/issues/1156
Starting with HDF5 version 1.10.x, the plugin code MUST be
careful when using the standard *malloc()*, *realloc()*, and
*free()* function.
In the event that the code is allocating, reallocating, or
free'ing memory that either came from -- or will be exported to --
the calling HDF5 library, then one MUST use the corresponding
HDF5 functions *H5allocate_memory()*, *H5resize_memory()*,
*H5free_memory()* [5] to avoid memory failures.
Additionally, if your filter code leaks memory, then the HDF5 library
generates a failure something like this.
````
H5MM.c:232: H5MM_final_sanity_check: Assertion `0 == H5MM_curr_alloc_bytes_s' failed.
````
This PR modifies the code in the plugins directory to
conform to these new requirements.
This raises a question about the libhdf5 code where this
same problem may occur. We need to scan especially nc4hdf.c
to look for this problem.
I took Ed's advice and moved the plugin stuff to its own
top-level directory. This is an attempt to solve the problem of
copying files that we have experienced. In any case, it will
serve as a place to stick additional plugins.
