Cleanup - replacing log-gamma function with F2008 intrinsic in WACCMX code

### Issue Type

Code Clean-up

### Issue Description

We had an older, outdated way of computing a log-gamma value. This switches it to use the F2008 intrinsic.

I ran one test with the change and it passed, but I've held off from all tests until Francis or someone else signs off, since this is WACCMX code. Mathematically, the function and intrinsic should be the same, but I could envision there being small numerical differences resulting from this.

### Will this change answers?

I Don't Know

### Will you be implementing this yourself?

Yes