Enabling Safety Checks by Default

[willtebbutt]

I need to find a loud way to get safety checks turned on by default in upstream packages, so that users make use of this functionality by default, and so that it's obvious to them how to turn it off when they're confident that they don't need it.

This requires:

• updating ADTypes to support safe mode Add safe mode SciML/ADTypes.jl#55
• updating DifferentiationInterface to respect safe mode
• updating LogDensityProblemsAD to respect safe mode
• printing messages to inform users that they're operating in safe mode -- see Improve safe mode options #149