Add a more convenient way to set aux arrays

[ketch]

Setting the aux array is somewhat different from setting q because boundary values must also be set on initialization. Right now the user is required to create a numpy array of the appropriate size and fill it in. Aron suggested providing a simpler interface that would only require the user to provide a function setaux(x, y, dx, dy) that takes arrays of cell centers and dimensions and returns an array of aux values for the cells. Then the grid would have a method that applies this function automatically using the coordinates of the grid. It was agreed that this would be helpful and should be supported, but without removing support for the current approach. The grid could also have a method to impose common "boundary conditions" to generate the ghost cell aux values (zero extrapolation and periodic).