Convert an array index from real to int, satisfing nag compiler

[samsrabin]

Description:

I'm working on a CTSM version compatible with the luh2_nocomp_merge branch, and I decided to run it through the nag compiler we use for testing. I got one error, about an array index not being an integer. This one-line change fixes it.

Collaborators:

None

Expectation of Answer Changes:

None

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the in-code documentation .AND. (the technical note .OR. the wiki) accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] FATES PASS/FAIL regression tests were run
• [ ] If answers were expected to change, evaluation was performed and provided

[ckoven]

Thanks @samsrabin!