Reduce example run times

[Jason Vander Heiden]

We need to reduce the example run times for testMRCA() in v0.2.5:

Examples with CPU or elapsed time > 10s
              user system elapsed
plotMRCATest 16.10   0.02   16.11
testMRCA     14.73   0.00   14.73 

Probably by redoing the algorithm. I think we don't need to redo the network search with each permutation. Instead, we can walk the graph once, store the node distances, and redetermine the MRCA from that info with each permutation.

[Jason Vander Heiden]

Not worth the trouble. Wrapping in donttest{} is sufficient.