Ensure vtkAxisActor labels are scaled similarly when using TextActor3D mode

When using vtkAxisActor with UseTextActor3D set to True, the scaling of each axis label is potentially different. If a label is for example, '1', it will end up appearing very small relative to the other labels on the axis due to the way label scale is currently handled. The proposed approach alleviates this issue by calculating a maximum label scale to use across all labels, and then applying it to all the labels. This fix is particularly important when control over the axis labeling font type is desired, or when unicode characters are necessary as part of the titles or labels - the default vtkVectorText cannot handle either. See attached screenshot.