permit non-square grids

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.)