Halo circular velocity function from Maller & Bullock 2004

[sarahbridle]

Description

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

[rrjbca]

Great start! A few suggestions to improve the code:

• We prefer to avoid abbreviations in variable names where practical, so M should probably be renamed mass
• We try to use astropy units where appropriate. So you should do from astropy.units import Unit and then e.g. 96.6 * Unit('km s-1') and (mass.to_value('Msun') / 1e11)
• Use cosmology.h rather than cosmology.H0 / 100. This also has the correct units.
• We need a test for the function. See e.g. test_carroll for a simple example of writing tests. Create a new file skypy/halo/tests/test_properties.py and write a function test_circular_velocity. An easy test would be to choose the arguments such that each bracket in the function equals one and check that the returned value equals 96.6 km/s. Perhaps you can think of others
• There are some trivial formatting issues relating to whitespace/tabs/blank lines etc. At the bottom of the pull request next to "codeclimate" click "details" to see the suggestions

[Lucia-Fonseca]

It needs some changes and tests.
Please, take this review as suggestions.

[Lucia-Fonseca]

We should update the description to link with the correspondent issue.

[itrharrison]

Replaced with #529 to change target branch.