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).