Bug: dataset.times method does not work

at the moment, the dataset.times method does not work at all, see also comments added in !6 (4cae8366).

It worked in an earlier version of the code, and returned a list of Python datetime.datetime objects. I'd suggest to change that, and return a np.array of dtype datetime64[ns] instead. This is to

1. avoid some rough edges of Python standard-lib datetime
2. keep it consistent with the other data return types
3. simplify integration with libraries used downstream of the data processing pipline, which presumable will be numpy, pandas and xarray

We should also add tests to ensure that method works.