Implement calculation for overall addition limit #1428

Is your feature request related to a problem? Please describe

Now that we have capacity growth (#1424, #1425) and total capacity (#1417) constraints enabled, we should update ProcessInvestmentConstraint and its get_addition_limit method to calculate the overall addition limit, using the provided constraints.

Describe the solution you'd like

• The capacity growth limit (but not total capacity limit, I believe) will need to be pre-scaled at load time based on the gap between milestone years, just like we currently do for the addition limits.
• The capacity growth limit will need to take the growth seed into account
• The scaled capacity growth and total capacity limits should then be passed to get_addition_limit, where the maths to calculate the most restrictive addition limit should take place.

Some details on implementation here

Describe alternatives you've considered

None

Additional context

N/A