Better x axis labels for the strong scaling bar graphs, and also a legend.

Changed the weak scaling code to calculate the total number of cells per
thread. The graph produced has all the points clustered around a straight
line. The same relationship would have come out of the previous version,
but it would not have been immediately visible, and would only have become
apparent after doing some maths.
Merge branch 'master' of github.com:ryanleong/ZombieApocalypse