Update core.cpp
Small doc update
OLD lambert solver restored and numerical precision for small angles improved
initialGuessImprovedForLambertSolver
A new basic function is added to measure a DV necessary for a  fly-by from  vin, vout,pl
Merge pull request #4 from JohannesSimon81/master

A new basic function is added that measures the DV necessary for a  fly-by to happen
Merge pull request #1 from esa/master

Getting the latest PyKEP