- 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.