RigidSolver as parent class

[lcaquot94]

Hello,

Is there a particular reason not to use RigidSolver as a parent class? I see in the drone example that the solver used is directly RigidSolver and that this class implements very "drone" specific functions such as "_kernel_set_drone_rpm". I would propose a PR to reorganize things a bit, by implementing for example a DroneSolver class inheriting from RigidSolver. But before I start, I wanted to make sure there were no particular reasons for such a code structure.

[YilingQiao]

Thank you for the proposal. It sounds good to me!