Added capability to plot mesh boundaries on top of slice plots

The PR adds functionality to specify a new element in plot elements like:

```xml
<meshlines mesh="1" linewidth="2" color="0 255 0"/>
```

This will draw the mesh boundaries for mesh with id=1, which must be defined in tallies.xml, with the specified linewidth and color.

I've found this useful for debugging tally setup, so I figured I would submit a PR for it.  Currently it only supports plots of basis xy, but it shouldn't be hard to extend.