New, flexible LXe NR yields and resolution model + G4 improvements + linear Noise + much more

• Better nuclear recoil model, with all parameters free for both means and widths
• ER model now includes optional Velan weighting function for mixing betas and gammas in testNEST
• "Linear noise" model for both S1 and S2 for treating unknown systematics in energy resolution
• Digital, integer number of PMTs hits are now saved both before and after the coincidence requirement of N photons within X nanoseconds of each other.
• More robust calculation of Nex and Ni for NR for any future model, and more protections against flaws like negative recombination probability or excitons
• testNEST can be linked against in external code including python, since it has named internal function now not just "int main()"
• Geant4 integration vast improvements: pair production, positrons, gammas, process trigger function, position-dependent E-field within G4
• Faster random number generator (xoroshiro)
• Enriched Xenon differentiated from solid better (confusion was caused by density)
• Resolved return 0, 1 for no errors versus yes an error happened
• beta->NEST::beta for CentOS7 ambiguity problem
• Travis was not testing anything before running testNEST, but now it runs actual simulation
• README improved for python and custom projects