How to handle non uniform definition?

[twhughes]

right now user can supply array-like object for one of the dl in the grid_size tuples. This specifies the grid sizes of each of the cells. There are a few subtleties:

• need to check whether supplied array covers entire simulation size in that dimension.
• need to check whether ratio of adjacent dl criteria is satisfied
• where is dl array centered? not straightforward.
• Need to convert this to coords specifying bounds of each yee cell

Eventually need to come up with better way to define non uniform mesh, or routine for generating it. Perhaps by introducing dl_in_material or similar parameter, or making plugin that can generate a Grid object that we introduce.