Support 64-bit integer memory type

[wkliao]

This PR allows user programs to read/write 64-bit integer data of internal memory type (i.e. integer*8) to/from classic CDF NetCDF files. It was tested using the following NetCDF-C settings.

• --disable-netcdf4 --disable-pnetcdf
• --enable-netcdf4 --disable-pnetcdf
• --disable-netcdf4 --enable-pnetcdf
• --enable-netcdf4 --enable-pnetcdf

[edhartnett]

Wait, does this capability exist in the C library?

[wkliao]

In NetCDF-C header file, netcdf.h, such APIs have been defined. For example,

EXTERNL int
nc_put_vara_longlong(int ncid, int varid, const size_t *startp,
                  const size_t *countp, const long long *op);

[wkliao]

I believe this PR also addresses issue #62
The newly added test program nf03_test/tst_fill_int64.F90 does the checking.

[WardF]

Thank you very much, this is much appreciated!