Position-based battery failsafe

[wsilva32]

• Extended the position based failsafe math implemented earlier (but never before activated)
• Calculates rise, traversal, and descent distances for RTL based on the HOME and vehicle locations
• The resulting mAh required becomes an additional current failsafe penalty
• Lowered the baseline current failsafe back to 520mAh
• Assume that at worst case RTL, the vehicle will draw 26 amps
• Reduced RTL speed to 9 m/s to better accommodate the R10C payload

[wsilva32]

I'll flight test tomorrow and post comparison results.

[ghost]

Please don't merge this until we're ready for v4.2

[wsilva32]

@liamstask once flight tests have performed, we'll need to rebase prior to merge. Done.

[liamstask]

👍