CDF_UINT8 does not exist

[subagonsouth]

https://github.com/MAVENSDC/cdflib/blob/main/cdflib/xarray/xarray_to_cdf.py#L169
CDF_UINT8 does not exist. This should be cast to a int64 and then use CDF_INT8?

[bryan-harter]

If we're being super strict about it, then we'll just reject np.uint64 altogether and display an error that we can't handle that data type. I hesitate to convert anything automatically that has the potential to significantly alter the data