netcdf-c

Updated some links in Doxygen version of user guide.  Fixed some
range_error checks in netCDF-4 type conversion code.  Made netCDF
attribute tests with type conversion more comprehensive and stringent,
fixing bugs identified with better tests.  Changed a test in
nc_test/tst_atts.c to use netCDF-3 file instead of netCDF-4 file,
because that directory is supposed to be for tests that work with
--disable-netcdf-4.  Added test demonstrating NCF-171 bug on 32-bit
platforms, only run when configured with --enable-extra-tests.