Corrections for helperfunctions

Now passes uppg1
Ninja
Script for ISS orbit
Testcases
Ninjacommit
Uppg4
Calculate linear momentum in one timestep
Calculate linear momentum for several timesteps
Function for calculating kinetic energy
Adds calculatePotentialEnergy for uppg4
Epic ninjas

Don't ieven sure what works anymore
Calculates orbital times
Data for orbittimes as comment
Merge remote branch 'asheidan/master'
Adds introduction, fixes latex-typo, adds repo url