Fix parametric young stars

[christopherlovell]

Updates the parametric young stars implementation to create individual star particles on the grid. This is opposed to the previous implementation, which was implemented on generate_lnu and directly produced the spectra from a parametric stars object; this led to issue with emission models with age dependencies, e.g., the CF00 dust model.

In this implementation new stars are created on the grid. These can then be self consistently treated as normal star particles for all downstream tasks. The old stars are saved on the object, as well as details of the parametrisation. This means that the procedure can be reversed, or new parametrisations applied, relatively easily.

This relies on the add_parts branch, and makes some additional modifications to this to generalise addition of particle stars object array attributes.

Additional changes:

• renames particle.stars.sample_sfhz to sample_sfzh
• fixes a bug in generate_lnu where fesc could be equal to None
• adds checks for nan or inf on particle stars input (ages, metallicities, initial_masses)
• adds properties on particle stars object for integrated total mass and SFR (optional timescale)
• fixes shape issues in some measure_ methods on sed

Issue Type

• Enhancement
• Bug

Checklist

• [x] I have read the CONTRIBUTING.md -->
• [x] I have added docstrings to all methods
• [x] I have added sufficient comments to all lines
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no pep8 errors
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes