re: https://github.com/Unidata/netcdf-c/issues/2189
Compression of a variable whose type is variable length
fails for all current filters. This is because at some point,
the compression buffer will contain pointers to data instead
of the actual data. Compression of pointers of course is meaningless.
The PR changes the behavior of nc_def_var_filter so that it will
fail with error NC_EFILTER if an attempt is made to add a filter
to a variable whose type is variable-length.
A variable is variable-length if it is of type string or VLEN
or transitively (via a compound type) contains a string or VLEN.
Also added a test case for this.
## Misc Changes
1. Turn off a number of debugging statements
re: https://github.com/Unidata/netcdf-c/issues/2117
re: https://github.com/Unidata/netcdf-c/issues/2119
* Modify libsrc to allow byte-range reading of netcdf-3 files in private S3 buckets; this required using the aws sdk. Also add a test case.
* The aws sdk can sometimes cause problems if the Awd::ShutdownAPI function is not called. So at optional atexit() support to ensure it is called. This is disabled for Windows.
* Add documentation to nczarr.md on how to build and use the aws sdk under windows. Currently it builds, but testing fails.
* Switch testing from stratus to the Unidata bucket on S3.
* Improve support for the s3: url protocol.
* Add a s3 specific utility code file: ds3util.c
* Modify NC_infermodel to attempt to read the magic number of byte-ranged files in S3.
## Misc.
* Move and rename the core S3 SDK wrapper code (libnczarr/zs3sdk.cpp) to libdispatch since it now used in libsrc as well as libnczarr.
* Add calls to nc_finalize in the utilities in case atexit is disabled.
* Add header only json parser to the distribution rather than as a built source.
