[FEATURE] some explicit args in open_tslong, open_tswide, open_tsdf and wrap methods

[martibosch]

Currently, many methods (see the issue title but I may be forgetting others) accept args and kwargs that are then forwarded to a backend-specific method, e.g., from_tstore. However, some arguments, e.g., time filters, are likely the same in all backends, so we should add them explicitly to the methods' signatures (and add docstrings).

Additionally, I would suggest that we use keyword-only arguments as much as possible, since they enforce good pythonic practices (i.e., explicit better than implicit) and make backwards-compatibility much easier for new releases. Related: #21