Note that this is a major, API-breaking change.
Also bunched into this commit:
- rename namespace `shortestPaths` to `ksp`
- omit unneeded namespace qualifiers
- move tests from `GraphTest` to `KSPTest` and wrote more
- path representation explanation in .md file
Former-commit-id: f395c3df40 [formerly 7fa07d1c8f]
Former-commit-id: f50dd3ce7c