This is the first step in a longer series of changes I would like to make to mplot3d. The first commit is meant to consolidate the hard-coded values used in axis3d objects into a single user-modifiable data structure. Note though that changing any particular value does not necessarily mean that that value would result in a change in appearance (such as pane colors, which has to be set through the axis object's set_pane_color()).

Furthermore, some properties may not immediately correspond intuitively to what is visually seen. For example:

ax.x_axis.set_pane_color([1.0, 0.0, 0.0, 1.0])
ax.y_axis.set_pane_color([0.0, 1.0, 0.0, 1.0])
ax.z_axis.set_pane_color([0.0, 0.0, 1.0, 1.0])

produces a red pane for the yz-plane (upper-left), green pane for the xz-plane (upper-right), and a blue pane for the xy-plane (bottom). This is correct, but would probably be best handled through documentation of a axes3d method that would explain which panel gets colored.

I will see what other changes can be done to this pull request within the next week, but when it comes time to put out the next release, feel free to merge the changes that have been made so far to master.