Kinetic energy computed incorrectly

[austinschneider]

It seems that the kinetic energy is computed incorrectly in the Particle class.
Thankfully this does not appear to be used anywhere in the library, and only seems to be referenced in one of the examples and the marsum executable.

marley/src/Particle.cc

Lines 99 to 101 in 9889611

	double marley::Particle::kinetic_energy() const {
	return std::max(four_momentum_[0] - mass_, 0.);
	}