rename file format strings

[Jonathanlyj]

rename 64BIT_OFFSET to NETCDF3_64BIT_OFFSET
rename 64BIT_DATA to NETCDF3_64BIT_DATA
add NETCDF3_64BIT_CLASSIC option, equivalent to None

[wkliao]

I suggest we rename 64BIT_OFFSET to NC_64BIT_OFFSET
and 64BIT_DATA to NC_ 64BIT_DATA, so they appear the same
as the C constant.

In addition, we also add the ones used in NetCDF4-python.
So, NETCDF3_64BIT_CLASSIC is the same as NC_64BIT_OFFSET
and NETCDF3_ 64BIT_DATA is the same as NC_ 64BIT_DATA