Improve performance of 3D displays with agent labels

[etatara]

3D Cartesian displays with agent labels render slower in RS 2.9 compared with version 2.7. A change in the jogl / J3D implementation is suspected. We needed to include a fix for a font render issue with RS 2.8 and 2.9 as described here: #51 and this may possible be related.

Java/Jogl had a major update so this is likely the culprit as the Repast label code has not changed significantly. It appears that in the method AbstractLabel.getBranchGroup() there is a lot of label related object destruction and creation. Perhaps we can preserve the label objects and simply translate them instead of creating new instances on each update. This might speed up label updating.