Support int64/uint64 enums?

[czender]

NcML 2.2 does not not appear to support an enum8 type that would properly store keys with base type int64 and uint64. I say this because the 2.2 schema omits any mention of enum8, and when I try to read a file containing a uint64 enum (which netCDF C supports) into toolsUI it barfs. Please implement enum8 and/or suggest a workaround. A file containing a uint64 enum is here.

[DennisHeimbigner]

Some years ago, John Caron and I had difference of opinion about
supporting enum8. I was for it, he was not.
I no longer remember the details of the pros and cons, but it
was probably about the semantics of enum constants.

We can add it, but it is would take a while because it will
affect the code in a large number of places.