Document impact parameter mapping

[fjebaker]

Update the getting started guide and docstrings to reflect the definition of impact parameters

$$ \alpha := \frac{ p_{(\phi)} }{ p_{(t)} }, \quad \beta := \frac{ p_{(\theta)} }{ p_{(t)} } $$

and how they are mapped from an (stationary) LNRF.

[fjebaker]

Included here should be a demonstration of the issues in comparing first order method trajectories with second order method, especially when the observer is close to the central singularity. The error in disc size then goes as $\sim r_\text{disc} / r_\text{obs}$