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)