re: PR Unidata#2088

Primary changes:
* Add NCZarr-specific quantize function to the dispatch table.
* Copy quantize code from libhdf5
* Add quantize invocation to zvar.c
* Add support for _QuantizeBitgroomNumberOfSignificantDigits to ncgen.
* Copy quantize test from nc_test4 to nczarr_tests. Remove some parts that are not relevant to NCZarr.

Other Changes:
* Break zsync.c into zsync.c (writing) and zload.c (reading).
* Clean up the fill value handling (many changes)
* Disable atexit() under Windows
* Move ncjson to libdispatch
* Add documentation of differences between netcdf-4 and NCZarr, especially WRT fill value.
* Some mingw fixes
* Remove some cruft
* Cleanup the handling of scalars

re: PR Unidata#2140

Fix some outstanding problems with the above PR.
* Minor consistency change to --enable-libxml2
* Simplify the makelib construction rule in libncxml/Makefile.am by eliminating the need for unistd and the file apis.

…df-c into zarrbg.dmh