Implement flexible period length

[willu47]

Users are currently restricted to running models with consecutive annual time periods.

Model size could be more effectively managed with little loss of insight if model periods were flexible. For example, instead of 2020, 2021, 2022, 2023 ... 2045, 2046, 2047, 2048, 2049, 2050; you could define key years 2020, 2025, 2030, 2040, 2050.

An advanced example of this is the TIMES formulation where there is a separation between data years and reporting years.

This paper implements 5-year time steps in OSeMOSYS model of Egypt.

The main issues for formulating this are:

• Cumulative bounds
• Discounting should be reasonably okay, but this may need to be adjusted if working with reporting periods of variable length