SITL: Put SIM::Battery's temperature update into a function

[hunt0r]

Summary

Simple change: Put a block of code into its own function.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

This is covered by SIM::Battery's unit tests.
For fun, I added an incorrect change to the new code and verified locally that the test fails as expected.

[peterbarker]

LGTM