rof month_annual_flow.ipynb has empty slice error

[TeaganKing]

Describe the bug
In the rof month_annual_flow.ipynb, there is an empty slice error resulting from "inconsistent observation data length across the sites (some sites provide the data for a short period). No observed flow data is available depending analysis (or simulation) periods." (@nmizukami)

To Reproduce
Run the month_annual_flow.ipynb in the coupled models example.

Expected behavior
It'd be great to resolve this error with some if statements that could adjust the expected time period over which the calculation is done.

Additional context
Naoki was planning to adjust this in another PR.