Suppress need for `MIN_START_YEAR` and `MAX_START_YEAR` parameters to match supplied initial populations

[andrewbaxter439]

Branching off from #196 - is this worth a brief fix?

Skimming through issues to check if anything I can do to help tidy up. This one could be closed but giving a suggested, more focussed issue:

• Suppress need for MIN_START_YEAR and MAX_START_YEAR parameters to match supplied initial populations

The reasons for this would be:

• Not all starting populations from 2011 to 2021 are needed for every run
• Changing this currently needs either adjust parameters/recompile, or setting in yaml (neither of which are inherently very tidy)
• Detecting files and dynamically setting min/max would suit all use cases?

Originally posted by @andrewbaxter439 in #196