main.cpp: Takes filename from command line.
.gitignore: Ignored datafolder as well.
Got rid of nan error.

System.cpp: Error was due to too many skips in force calculation since
the if-test used the wrong operator. Was therefore true more times than
wanted.

Wrote new test, will now create Python plotter and see if data is
correct.

TODO: If this works and data is correct Leap frog method can be
implemented.