Distance to ZMaxPlane along a particular direction

@gcorti @admorris @mkmiec

This provides an extension of the standard ZMaxPlane class for fast simulation studies.

ZMaxPlane (or RhoMax in cylindrical calorimeters) is a special optimization applied in Gaussino and Gauss that limits the number of MCParticles persisted to storage. In particular, all MCParticles with vertices behind the ZMaxPlane will not be kept.

The concept of ZMaxPlane is crucial in fast simulations, and in particular in the CaloChallenge setup !146 (merged) for planar calorimeters. In this setup, the so-called collector plane and the actual zmax plane might not necessarily be in the same place, and therefore the distance that a particle travels between must be known.

In order to get this distance x from a point P, I use the following formula:

x = \frac{|x_{zmax}|}{\hat{n} \cdot{} \hat{v}}

where:

• x_{xmax} is the shortest distance to the ZMax plane from the point P (already provided by the functionality of the ZMaxPlane class,
• \hat{n} is the normal vector of the ZMax plane,
• \hat{v} is a vector describing the direction in point P.

I added one unit test to test this feature on an example.