Launch beam refactoring

[ZedThree]

(WIP until #74 is merged)

Pulls out the beam launch code into a separate functions and a new file, scotty.launch.

Fixes a bug where the toroidal launch angle was not taken into account for finding the launch entry position. This may affect simulations with large toroidal launch angles.

Enables launching from the inboard side (although more changes are required for this to work properly, namely the derivative stencils).

Still to do:

• [x] Add more docs

[valerian-chen]

Looks great. After you've completed this, I'll implement general non-circular beams at launch. That is, for arbitrary Psi_w_initial

[ZedThree]

@valerian-chen I've added some more docs and rearranged things a little bit, so this is ready for review now