Basic 1D finite difference code generator

[jrmaddison]

Arbitrary order centered finite differencing. Currently unrolls the expression. Differencing transitions, becoming increasingly one-sided as boundaries are approached. Now used by Grid in Grid.D_x, Grid.D_y, Grid.D_xx, and Grid.D_yy.

[jrmaddison]

Note that this uses a very basic linear solve based method to find the coefficients. Results are correct so long as everything is computed symbolically (diff_bounded uses exact rationals when finding the coefficients) but the method in doi:10.1090/S0025-5718-1988-0935077-0 would be much better if inexact arithmetic is needed. SymPy provides an implementation of the latter algorithm in sympy.calculus.finite_diff.finit_diff_weights, but it's unclear from the SymPy documentation whether a specific displacement ordering is needed, so that isn't used in this PR.