Mean output flows

[SouthEndMusic]

Fixes #935.

[SouthEndMusic]

To do:

• Incorporate Support only fixed saving intervals, set default at daily #1155
• Add test of mean flow output

[SouthEndMusic]

To do:
- [ ] Remove horizontal flows from flow output

• Update docstrings and documentation

[visr]

Would it not be simpler to return t_end (or perhaps [t_end]) here instead of filtering the 0.0 out for the SavingCallback?

[SouthEndMusic]

I don't know whether that's desirable since saveat is also used for the save subgrid level callback.

[visr]

Yeah I was thinking for that we can use save start = true

[visr]

Left some optional final suggestions.