Rail Buttons Force Calculation

[giovaniceotto]

There is no current way to determine the forces and moments on the pair of rail buttons which guide High Powered Rockets in the launch rail.

This could probably be implemented in the post processing phase of the flight simulation, in the Flight.postProcess method.

[Gui-FernandesBR]

Currently there are evaluations for Normal and Shear Forces on Rail Buttons,
It's lacking only bending moments calculations.
Altought I don't see too many uses of such values, I believe providing it to final user will be fast, simple, and beautiful. Then I'm marking it as a good first issue.

ps: Perhaps in the future we can create a place on rocktpy to be dedicated to structural analysis only.