Support for nested grids

[jimc101]

Many models are run in nested configurations, but PyLag doesn't support nested inputs. One way support for this could be implemented is through a new class that manages multiple, nested ocean datasets. A log could be kept of which nested product a particles resides within, and then field variables drawn from that. To check which nested product a particles resides within, one could form a polygon form the boundary nodes and check to see if it resides within the polygon. Each nested product would have its own grid metrics file and this would need to be reflected in the run configuration file.