Axis labels have been added

[cklotte]

Axis labels for both x-bins and y-bins were added, including tick marks for all of the axis labels.

Note on a thing to fix: When you configure the font size to very large, the 'density' label gets pushed out of the diagram, while the 'distance' label doesn't.

The next thing to do would be to implement a convention to determine the precision of which the labels are rounded to.

[mlwright84]

These axis labels are a nice improvement to the RIVET visualization! Overall, I like this a lot. However, I see a few issues that I would like to see improved before we merge this pull request.

1. If the user increases the font size, then the axis labels may extend outside of the viewable region, like this:
   
   This problem might have existed before the changes in this pull request, but it makes sense to fix it now.

2. If the user adjusts the window bounds using the top/bottom/left/right selectors, the max and/or min value isn't labeled in the window, even if there is enough space for the label. See the following example:
   

3. The numbers are sometimes printed incorrectly, as in the following example:
   
   Here, the printed labels don't agree with the coordinates that appear in the popup boxes when the mouse is hovered over the green and red dots. This was produced with the file data/Test_Point_Clouds/circle_data_240pts_codensity.txt, using 20 bins in each direction. Note that the smallest and largest bins in each direction are correct. For example, the lowest x-bin is 5.25, and the largest x-bin is 48.