gh-1112: distribute redshifts over shells

[ntessore]

Description

Add a new function glass.distribute() that takes an array of redshifts and distributes it over the given shells.

To achieve this, a new multinomial() distribution was added to the RNG interface.

Closes: #1112

Changelog entry

Added: glass.distribute() to distribute an array of redshifts over shells

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[paddyroddy]

Some thoughts