Include JNPE_t.hh

In analogy with JPDF_t.hh, a new include file JNPE_t.hh has been prepared. This include file contains the time integrated PDF tables, commonly referred to as NPE tables. These tables are useful to evaluate -for example- the hit/no-hit probabilities for PMTs in the detector. Note that the constructor JShowerNPE_t has a second argument; when the specified number of points is larger than 0, the longitudinal profile of the shower is taken into account on-the-fly. An example is available at <Jpp>/examples/JPhysics/JShowerNPE[.sh]. Note that for the sake of plotting, the histogram is normalised to the energy (i.e. unit is npe/GeV).