The code now runs for Mercury! Yay!

Changed line 135 in system.cpp (angular momentum per mass) and line 109
in vec3 (vec3's are printed with higher precision).
Added a printing function which only prints at each n'th time step,
where n is an input parameter.

Commented out the Mercury part of main, since we now want output from
the Sun-Earth-Jupiter system or the "entire" solar system (all planets
plus Halley's comet and some moons).