SITL: modifications to lift in SIM_StratoBlimp

[Dan-field]

• Update SIM_StratoBlimp.cpp
• Update SIM_StratoBlimp.h

Modification to how lift is simulated in SIM_StratoBlimp.

The aircraft itself is always neutrally buoyant.
A 'helper balloon' is attached at initialisation. It provides additional lift according to the Free Lift Rate (variable). The helper balloon detaches once the target altitude is reached.

[Dan-field]

1 unsuccessful check was sitltest-sub with the following reason:

FAILED STEP: test.Sub at Tue Apr 30 03:46:12 2024
Failure Summary:
test.Sub:
SimTerrainSurftrak (Move at a constant height above synthetic sea floor) (Distance not maintained: want 15.00 (+/- 1.50) got=13.21 (1.79)) (see /tmp/buildlogs/ArduSub-SimTerrainSurftrak.txt) (duration 8.376605033874512 s)
FAILED 1 tests: ['test.Sub']

This failure does not appear to have any causal link with the changes in this PR.

UPDATE
passed on second attempt after rebase