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 meshtype="tally" id="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.

Other mesh types can also be drawn instead like:

```xml
<meshlines meshtype="entropy" linewidth="2" color="0 255 0"/>
```
or
```xml
<meshlines meshtype="ufs" linewidth="2" color="0 255 0"/>
```
or
```xml
<meshlines meshtype="cmfd" linewidth="2" color="0 255 0"/>
```

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.~~

TODO:

- [x] Support the other slice directions
- [x] Support plotting the entropy and UFS meshes
- [x] Support plotting the CMFD mesh

I was thinking the same thing when I looked back at the heuristic; it probably would make more sense being in input_xml. If you're feeling ambitious, you can move it as part of this PR. Otherwise we'll move it later.