Update PR template to include request for benchcab outputs

[bschroeter]

In order to expedite code review, the onus should be on the code author to provide test results that the code change still permits CABLE to compile and run without error. The simplest way to do this is to include a benchcab step with bitwise comparison, as it will cover both situations.

A simple update to the PR template should be sufficient to encourage authors to get in the habit of testing with benchcab prior to requesting a review.