Transit duration not perfectly matching batman

[hippke]

Fixed: Bug was here:
Planetary transit duration at b=0 equals the width of the star
Formally correct would be: (R_star+r_planet) for the transit duration T1-T4
Here, however, we need T2-T3 (points where center of planet is on stellar limb)
https://www.paulanthonywilson.com/exoplanets/exoplanet-detection-techniques/the-exoplanet-transit-method/
New:
tdur_p = per_planet / pi * arcsin(sqrt(R_star ** 2) / a_planet)

Verification with compare_to_batman.py at very high time resolution. All differences are <1e-7 now.