Add Haser filling factor

[mkelley]

This is a request for

• a new feature
• an enhancement to existing sbpy functionality
• somethings else: [explain here]

The requested changes will be implemented by

• me
• the sbpy developers

High-level concept
Some investigators calculate the Haser filling factor: the fraction of coma molecules expected within an aperture, e.g., f(x) in Eq. 1 of Bonev et al. 2017 https://scixplorer.org/abs/2017AJ....153..241B/abstract

Explain the relevance to sbpy
It is for comets!

Example (pseudo-)code

h = Haser(1e28 / u.s, 0.3 * u.km / u.s, 5e5 * u.km)
h.filling_factor(1e4 * u.km)