nxdataset size reporting limited to 32bit ints

**Original reporter**: _[zjttoefs](https://github.com/zjttoefs)_

While createNXDataset uses 64bit ints for indexing:

```
pNXDS createNXDataset(int rank, int typecode, int64_t dim[]);
```

reporting works on architecture ints:

```
int   getNXDatasetDim(pNXDS dataset, int which);
int   getNXDatasetLength(pNXDS dataset);
int   getNXDatasetByteLength(pNXDS dataset);
```
