Allow specification of machine params from the command line

## Problem

Currently it can be a bit of struggle to specify machine parameters as you have to manually change parameters in the configuration.

## Solution

Let's have a command-line means by which to change machine parameters. Some things that would be nice to be able to change:

* Workers and their devices
* # Samplers and their devices

One possible problem is that we might need a way to change the above for training/validation/testing separately. This might be a little messy.

## Dependencies

None.