generateMuons creates muons coming from the vertex flying in direction  phi in r/phi plane with inclination lambda. The particles are MCParticles for use as input for the PrimaryIonisationProcessor.

Usage:
generateMuons energy phi lambda nEvents [outfile.slcio]

* Use 0..2pi to define a fixed phi angle. Negative phi gives random angle.
* Lambda range is -pi .. pi
* Outfile name is optional,  default is MCMuons.slcio
* Energy is in GeV