permit non-square grids

[davidwhogg]

I like putting the nodes on an integer grid, for consistent-style reasons (for example, I want to do that in Data Analysis Recipes). But sometimes you might want that grid (in increasing order of difficulty):

• rectangular (think neural networks where the vertical spacing is always much larger than the horizontal)
• triangular (think again neural networks or (even better) equilateral triangular)
• other geometries (like polar etc)

This is an enhanced goal because we don't need this for present goals. (For some reason I don't have the right to label the issues with tags.)