implement functionality to generate temporal masks from conditions

This functionality is inherited from `evalhyd-cli`. It allows the user
to provide conditions as strings to specify how to generate temporal
subsets. Conditions can be based on observed streamflow values (e.g.
q>800, q<=120) or on time indices (e.g. to select particular events).

This functionality is made available both for determinist and
probabilist evaluation, unlike in  `evalhyd-cli` where it was only
available for probabilist evaluation.

This is documented in the docstrings, and new unit tests are written.