Support bounds for mrmatrix

pete says:
> In the future, it should work like in [`npmatrix`](https://github.com/higlass/clodius/blob/37b304ac12f7d8ed0209138b549c4e03ecdf9c8d/clodius/tiles/npmatrix.py#L27). The reasoning: a matrix, as implemented programmatically is necessarily a 2D array whose indices go from (0, 0) to (len-1, len-1). Imagine that we want that matrix to represent rasterized values for a geographical region. Bounds would indicate the upper left and lower right corner latitude, longitude pairs for the rasterized region. So you could have a 1000,1000 matrix represent a region that goes from (10 degrees,10 degrees) to (10.1 degrees, 10.1 degrees), for example.