Add functions to calculate temperature of particles.

Changes:

• Added function to compute T for a given particle and cooling model (#511 (closed)),
• Used the correct definition of Helium fraction throughout EAGLE cooling code (#510 (closed)),
• Added temperature to the snapshots
• Fixed a mistake in the EAGLE Compton cooling code that was using logT and not T.
• Style improvements to the EAGLE cooling code.
• Fixed the EAGLE examples yaml files to use the correct name for cooling-related variables.
• Clarified calculation of delta redshift for a given time-step size.

changed the description

merged

mentioned in commit 3e0b00c6

mentioned in issue #511 (closed)

mentioned in issue #510 (closed)