Expose Elara's version via the CLI

[mfitz]

Elara's CLI currently exposes the following subcommands:

Usage: elara [OPTIONS] COMMAND [ARGS]...

  Command line tool for processing a MATSim scenario events output.

Options:
  --help  Show this message and exit.

Commands:
  run              Run Elara using a config.
  event-handlers   Access event handler output group.
  plan-handlers    Access plan handler output group.
  post-processors  Access post processing output group.

We should add a version subcommand, and/or support for elara --version/elara -v, as a way to report the version number, which we currently record in the elara/init.py module, although it seems to be neglected.

Adding support for displaying the version via the CLI will also drive better and more organised/formalised versioning and release behaviour in general, including semantic versioning, a well maintained change log, and tagged releases with release notes on GitHub.

Once we are on top of versioning, we could also subsequently add a feature or extend the version subcommand such that Elara checks your local version and prints a message to console if a newer version is available. That would be covered by a separate issue to which this issue would be a precursor.