Add plate cooling to adiabatic initial temperature plugin

[danieldouglas92]

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[danieldouglas92]

I addressed most of the comments, but the test case that I've written does not have a varying temperature. The temperature everywhere in the model is equal to the adiabatic surface temperature set in the input file (for both the half space and plate cooling model, and for ages read in from an ASCII file or set in the input file)