MGA module: add new constructors

[MiguelAvillez]

Hi all,

It would be nice if a couple new factory functions were added for the creation of MGA transfers:

• Directly define an initial/final state for the transfer: currently the initial/final state comes from the state of the departure/arrival body, it should also be possible to select initial/final states not associated with any body. Could be useful for validation purposes, e.g. to compare with the thrust arcs of some mission (@sbcowan 😁 ). Right now this can be circumvented by defining the departure/arrival bodies with user-specified constant ephemerides and a dummy gravitational parameter.
• Allow the definition of departure/arrival orbits e.g. for Earth-centered transfers. Right now this can be circumvented by defining the departure/arrival bodies with the desired orbits and a dummy gravitational parameter.