Added --collapse option to parse_exps.py for creating simpler graphs.

This creates csvs which have merged the lines which only differ by variables
whose values are numbers (e.g. 'wss', 'number of tasks', or 'duration',
but not 'scheduler').

For example, consider a csv which is tracking the change in working set sizes.
There are 3 different experiment configurations per WSS, one with 12 tasks,
one for 14, and another for 16. Without --collapse, a seperate csv
(and line to plot in plot_exps.py) will be created for each of the 12, 14, and
16 configurations. With --collapse, a single csv (and a single line plotted
by plot_exps.py) will be created which averages the values of 12, 14, and 16
tasks for each WSS.

This is very useful for noticing trends or presenting overhead bar charts
or other examples in papers.