Automatic management of version numbers

[slivingston]

From the User's Guide, "The repository as a whole has a single version number. Depending on the eventual pace of growth and styles of usage, we may begin to version significant components separately." The ambitions of this issue:

1. automate management of version numbers, having at most one location in the repository in which the version is manually declared;
2. support generation of version annotation before releases, e.g., including Git commit hashes;
3. decide whether to version parts of the repository separately, e.g., particular benchmarks vs. common infrastructure like the fmrb Python package.