Add new subcommands for sim

[Autoparallel]

Now that we have the skeleton of two sims portfolio_sim.rs and uniswap_sim.rs we should just have two subcommands. I haven't looked into the exact details, but it would be good to have something like:

arbiter sim -portfolio and arbiter sim -uniswap.

To close this:

• Add subcommand/flag for portfolio so that arbiter sim -portfolio runs portfolio_sim.rs.
• Add subcommand/flag for portfolio so that arbiter sim -uniswap runs uniswap_sim.rs.