Add case for computing derivatives and vorticity
A new case file, derivatives.cpp, gives a means to compute derivatives of any field or to compute vorticity components. The vorticity calculation in Science is completely re-written so-as to also work for mapped grids. The derivatives file is built to compute secondary variables after a run has already been completed. This uses spins' built-in derivative toolkit allowing multiple variables to be calculated in parallel. A derivative field (one that is the derivative of another) is denoted by an underscore followed by the direction the derivative was taken in (ie. u_x is the x derivative of u).
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