As discussed in a netcdf meeting, convert NCZarr V2 to store all netcdf-4 specific info as attributes. This improves interoperability with other Zarr implementations by no longer using non-standard keys.
## Other Changes
* Remove support for older NCZarr formats.
* Update anonymous dimension naming
* Begin the process of fixing the -Wconversion and -Wsign-compare warnings in libnczarr, nczarr_test, and v3_nczarr_test.
* Update docs/nczarr.md
* Rebuild using the .y and .l files
The primary change is to support the use of a zip file as a
storage format. Simultaneously the .nz4 support is made obsolete
Use of zip requires the libzip support library, so a number of
changes to the build files (Makefile.am, CMakeLists.txt) are
necessary to locate and incorporate libzip. The nczarr_tests
tests are also changed to add zip testing.
Other changes:
* Make sure distcheck leaves no files around.
* Add some functions to netcdf_aux to export some functions of libnetcdf.
* Add a new error NC_EFOUND as the complement of NC_EEMPTY.
* Add tracing support to nclog and use it in libnczarr.
* Modify the zmap interface to support the writeonce semantics of zip.
* Create a new s3util.c to support a variety of S3 auxilliary functions.
* EXTERNL'ize a number of functions so they can be used in s3util.
* Add support for the S3 ListObjects CommonPrefixes mechanism
to improve search.
* Add experimental support for running nczarr X s3 tests against
the actual Amazon S3 cloud.
