Plot history length and turning off git describe message

1- added stderr=PIPE in the GIT describe in topo/__init__.py to direct git's messages to the pipe.
2- Added history length to the Plot Pannel GUI. The length control is tested using:
from topo.tkgui.plotgrouppanel import PlotGroupPanel
PlotGroupPanel.maximum_plot_history_length =xxx
where xxx above was tried with -1, 0, and various > 0 values. Note the parameter above is documented in the code.