Added an optional `atmosphereAlt` paramater in bodies's descrptions.
The flyby calculation now prevents a periapsis altitude below
the atmosphere limit.
- Grouped satellites's and SOIs' visibility managment in a
`SolarSystem.update` member function
- The visibility of the satellites and their orbits depends on the
distance to their attractor and not to them.
- Reimplemented 2D orbital mechanics functions in physics-2d.ts
and encaspulated inside a `Physics2D` namespace.
- Deleted physics.ts and moved the required functions into physics-3d.ts
Further cleaning and reimplemntation will be done on the physics-3d.ts
content.
- Forced consistency on error handling : every raised error throws
an `Error` object.
- Commented sections relative to 2D physics and sequence generation.
