Implement method to add rail buttons to a rocket

[giovaniceotto]

Currently, the Rocket class has no support for rail button characteristic input.

With the Rail Buttons Simulation Milestone in mind, a method such as Rocket.addRailButtons should be implemented allowing the user to define the axial and tangential position of two rail buttons used in the rocket.

[giovaniceotto]

I will start working on this issue and the milestone in general in the rail_buttons branch.

[giovaniceotto]

This has been implemented in the last commit.

Just to clarify, only the option to add 2 rail buttons, or a rail button pair, was considered. The reason behind this is that in terms of flight dynamics, only the last two rail buttons of a rocket matter. When more than two rail buttons are guiding the rocket, the situation is indeterminate.

Therefore, since the method wasn't really adding rail buttons but only setting the position of the rail buttons in terms of axial and tangential coordinates, the method was named Rocket.setRailButtons().

I will be marking this issue as closed then.