Quincey's fixes for NCF-56, "netcdf-4 can't read HDF5 scalar atts".
Also added ability capability for netCDF-4 to write and read NIL
values for string type attributes and variables, so these can be read
if used in HDF5 files.

Include are additions to CMakeLists files to reflect new tests.


git-svn-id: https://sub.unidata.ucar.edu/netcdf/trunk@3287 2e3060e4-1ed8-cf1f-9cc2-9586e844a556