Fix MPCD compiler warning

[mphoward]

Description

The compiler warning was being generated because of the use of GammaDistribution in kernel::srd_draw_vectors. This PR rearranges that function to remove the use of recursion.

Motivation and context

Code should compile without warnings.

Resolves #1844

How has this been tested?

Existing unit tests pass, and the compiler warning is gone.

Change log

* Fix MPCD compiler warning.
  (`#1845 <https://github.com/glotzerlab/hoomd-blue/pull/1845>`__)

Checklist:

• [X] I have reviewed the Contributor Guidelines.
• [X] I agree with the terms of the HOOMD-blue Contributor Agreement.
• [X] My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.