Setting `initial_mass` in a parametric galaxy has no effect when an sfzh is provided

[christopherlovell]

When creating a parametric stars component, if you set the initial_mass this has zero effect unless you're using functional forms for the SFH and Z distributions. This can be confusing for users when changed and it has zero effect on the spectrum.

We could either: 1) remove the initial mass from the initialisation, and only ask for it when using functional forms 2) scale the provided SFZH array by this initial mass value (if provided)

[WillJRoper]

I think I prefer the latter but I see an argument for the former as well since it just simplifies things 🤔