Templating GammaCalculator over deriv_t type

[the-florist]

Changes:

• Made the GammaCalculator class a templated class over deriv_t type, in the style of the MatterCCZ4RHS class.
• Put the instantiations of the GammaCalculator class in the initialData functions of KerrBHLevel, ScalarFieldLevel and ApparentHorizonFinder3DTestLevel, in logic statements that change the template type based on the requested order of derivatives, as for the MatterCCZ4RHS class.
• Note that the ApparentHorizonFinder instance of GammaCalculator was commented out, and that although I have altered these lines, I have left them commented out.