Add seed parameter for capacity growth

[dc2917]

Is your feature request related to a problem? Please describe

We have a capacity growth constraint, but without a seed parameter this is useless.

Describe the solution you'd like

• Add a seed parameter to process investment constraints input file
• Add this as an optional field of ProcessInvestmentConstraintRaw (defaulting to 1)
• Add validation
• Add tests of validation

Describe alternatives you've considered

None

Additional context

N/A