Use subfigures and subcaptions for figures 8.2 and 8.3

[bkmgit]

Problem

The second and third figures in the topologies chapter are composed of two separate figures each, and it maybe difficult to distinguish between the subfigures.

Proposal

Each of these figures should have separate subfigure labels and sub captions.

Changes to the Text

Use subcaption or similar package to add captions.

Impact on Implementations

None

Impact on Users

Make the standard easier to read.

References and Pull Requests

[cniethammer]

@bkmgit I created PR https://github.com/bkmgit/mpi-standard/pull/1 against your tikz branch splitting the images in preparation for sublabels. Feel free to have a look and pull this in.