Add total capacity constraint

[dc2917]

Is your feature request related to a problem? Please describe

As in MUSE1, we should add a constraint on total capacity (a hard upper limit on total capacity) for investments.

Describe the solution you'd like

• Add an extra column to process_investment_constraints.csv
• Perform sensible validation (i.e. in range [0, $\infty$))
• Allow missing values (i.e. no constraint)
• Store this as an extra field in ProcessInvestmentConstraint.
• modify get_addition_limit to solve all the maths to get an overall addition limit, taking into account the two different constraints
• This addition limit can then be used downstream just as it currently is (i.e. scaled by the commodity_portion, returned from max_installable_capacity, etc.)

Describe alternatives you've considered

None

Additional context

N/A