Add the size-weighted mixing rule

[emarinri]

This adds the size-weighted mixing rule along with its documentation.

A new section in the forcefield section of the documentation was added to describe the functional forms of the available mixing rules.

Description

A description of this mixing rule is here

How Has This Been Tested?

Tested with hydrofluorocarbons. No automatic tests for mixing rules have been implemented yet, so no test in this regard was implemented.

Post Submission Checklist

Please check the fields below as they are completed

• Suitable new documentation files and/or updates to the existing docs are included.
• One or more example input decks are included.
• Suitable tests were added to the test suite
• My name is in the contributor list at /Documentation/source/reference/acknowledgements.rst