ENH: gravitational waves artale rates

[itrharrison]

Description

Adds a new model for compact binary merger rates to the gravitational wave(s) module, from Artale et al (2020).

There are three models, depending cumulatively in each of stellar mass, star formation rate and metallicity. Each model is a simple logarithmic model with coefficients for the log of each independent variable and a constant.

The values of the parameters are fit to objects in the EAGLE simulation suite, with independent fits on simulation output at four redshifts: 0.1, 1, 2, 6. At the moment a simple linear interpolation between these points is performed for the coefficients as a function of redshift.

ToDo

• Finish implementing and documenting SFR and metallicity-dependent models
• Expand tests
• Decide on consistent treatment of units
• Improve treatment of redshift dependence (function decorators?)

Checklist

• Follow the Contributor Guidelines
• Write unit tests
• Write documentation strings
• Assign someone from your working team to review this pull request
• Assign someone from the infrastructure team to review this pull request

[Lucia-Fonseca]

@itrharrison could you please add ENH:(enhancement) at the beginning of this PR's title? Thank you.

[Lucia-Fonseca]

A few minor details, erase examples and question on private functions.