Tree production in physics class

Added first portion of tree production in physics class. This will be done ahead of histograms.
Improved tree handling in Pi0 physics class.

A two stage physics analysis is now implemented in the example PPi0 class. A basic tree is created containing only the calculated Missing Mass and Energy, along with the TC, incident photon energy, theta, phi and time for timing cuts. Histograms can be produced from this tree (at the same time) without interfering with the tree. This is done so that the histogram binning can be changed on the fly (as well as missing mass cuts and prompt/random timing cuts). All controls are still hard coded into the init section, however this will change to be user controlled (either via command line flags, or through a config file).
Changes to Charged-Neutral conditions (Particle reconstruct)

Setting up basic outline for improved "charged particle marking". This is a seperate parameter to full charged particle identification (ie dE_E cuts) which looks at the energy deposition in the charged particle detectors to mark a particle as charged. Due to experimental problems with individual detectors, this must be done in a way which is possible to completely ignore the information from an individual detector.
Updates to PEta class

Small updates to reflect changes in PPhysics class (HistFile for histograms to seperate from tree file)