Added beam direction fix 2 + added flat beam

Found a bug, similar to one I fixed before, where the beam-width would always be oriented in the same plane, in this case the x-y plane. Lets say you fire a beam in the x-direction, the spread of the initial position of the beam particles will currently still be oriented in the x-y plane. I've attached a before- and after- figure so you can see the difference. Before the FIX:

After the FIX:

I've also added the option for a flat beam, instead of a Guassian beam profile, in case you're interested in adding this to Allpix. I still have to add this to the README, but I was not sure it'd be added.