Make sampling time saving work with hdf5

The sampling_time is often stored as a datetime.timedelta object in the result. This is not currently serializable for hdf5. This MR fixes that and adds sampling time to the result read/write test.

There's also a typo fix in the docstring so that IDEs can understand the expected type.

changed milestone to %1.2.0

added Bug label

approved this merge request

approved this merge request

merged

mentioned in commit 8298687f

mentioned in issue pesummary#284 (closed)