SITL: fix SIM_BATT_CAP_AH

[robertlong13]

Previously did nothing. The only way to set the capacity was to set it within your model's json.

[peterbarker]

@robertlong13 how does this interact with someone setting the capacity in the json?

[robertlong13]

@robertlong13 how does this interact with someone setting the capacity in the json?

After the json is loaded, it sets the default for this parameter to whatever capacity the json set it to. So, unless you go out of your way to set this parameter, this PR doesn't impact anything (tested with Callisto).

[robertlong13]

@peterbarker reminder on this one. Fell off my radar too

[peterbarker]

Merged, thanks!