Implement Contour plots

[martin-schlipf]

Sometimes it can provide insight to visualize 3d data as a 2d slice. For this contour plots would be a nice feature. The simplest plot could just take a plane of the regular grid but a more throughout implementation could use scipy.interpolate.griddata to get any arbitrary points on a regular grid.
To achieve this, we should introduce a new class analogous to Series. We should decide whether to allow multiple contours in a single Graph or not. Contours could either be an independent field or members of the Series list.

[martin-schlipf]

Initial implementation for densities