Add option for GtGunTool that can generate with costheta/sintheta distribution

Add 2 options 'UseCostheta' & 'UseSintheta' for GtGunTool, now GtGunTool can generate with costheta/sintheta distribution.

here is an example:

gun = GtGunTool("GtGunTool")
gun.Particles = ["mu-"]
gun.EnergyMins = [10] # GeV
gun.EnergyMaxs = [10.] # GeV
gun.UseCostheta = True
gun.ThetaMins  = [-1]
gun.ThetaMaxs  = [1]
gun.PhiMins    = [0]    # deg
gun.PhiMaxs    = [360]  # deg

'UseCostheta' & 'UseSintheta' are set to False by default. Only when 'UseCostheta' or 'UseSintheta' are manully set to True, 'ThetaMins' & 'ThetaMins' indicate the range of costheta/sintheta. Note that 'UseCostheta' & 'UseSintheta' cannot be set to true at the same time.

We simulated 50000 events with the options shows above, here is the distribution of MXParticle.momentum.z: