Add eagle feedback test

Adds a new test which tests cooling + feedback with hydro schemes. This is basically as sedov but using the exact same units as EAGLE does, with the heating to 10^{7.5}K.

examples/Cooling/FeedbackEvent_3D/README.md

+Feedback Event
+==============
+
+This is a modified version of the SedovBlast_3D test, that is supposed to be used
+with the sub-grid model, i.e. with
+
++ Hydrodynamics
++ Cooling
++ Star formation
+
+This is here to ensure that we are handling the feedback events correctly in the code.
+
+We should not form any stars here, but ensure that flag is switched on just in case.
+
+This test emulates what the EAGLE model does to particles for feedback, i.e.
+
++ Heats a single particle to 10^7.5 K
++ Does _not_ switch off cooling
++ Runs to completion.
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