GammaGammaCandidateFinder Processor

Purpose: Assemble a collection of ReconstructedParticles consisting of pairs of photons consistent with known 
         particles which can decay to two photons, including 
         pi0 -> gamma gamma, eta0 -> gamma gamma, etaprime(958)0 -> gamma gamma.

Note that the same photon will often be a constituent of more than one GammaGammaCandidate. 
The "photon matching" problem of assigning individual photons uniquely to at most one GammaGammaParticle 
or similar composite particle is planned to be treated in a separate processor after the creation of 
these GammaGammaCandidates.

trunk v>=5283
set nominal angle error to 1 mrad/sqrt(E (GeV) (was 1 mrad).

trunk v>=5196
check fit covariance matrix dimensions (sometimes 0 ...)

trunk v>=5186
Gamma-gamma four-vector (px, py, pz, E) output covariance matrix now calculated and filled.

trunk v>=5052
Include mass-constrained fit (and MarlinKinfit dependency). 
Fit probability currently stored in GoodnessOfPID, and gamma-gamma 4-vector is now fitted.
Implementation still needs work on handling covariance matrix from input - and covariance matrix on output.

trunk v5021 
Currently the algorithm does not do any mass-constrained fitting, but this is one of the main reasons for 
this processor, and the eventual GammaGammaCandidate collections will be mass-constrained.

An example xml steering file is examples/example_GGFinder.xml

Graham W. Wilson, 10th July 2015.