Allow 64-bit dimension lengths

[cwardgar]

From LQO-782179.

The length of a dimension in the CDM is limited to a signed int (32 bits). Presumably, this is to match the dimension length limit in NetCDF-3, which is also a 32-bit int (although it is unsigned, not signed). This limit has been in place at least 10 years, and probably appeared in NetCDF-Java 1.0.

Since then, NetCDF-4 was released, which supports 64-bit unsigned integer dimension lengths. We ought to upgrade the CDM to use type long for dimension lengths, which is a 64-bit signed integer.

[cyrilleponcelet]

Note that in java 8, some support for unsigned long where added to the class java.lang.Long, so we can use long type to be treated as unsigned