Add workspace context

[mrchtr]

The workspace will be used to hold all relevant execution information, e.g. name, base_path, run_id, etc.
It has still to be decided where and how to initialise the workspace.

Add cli command to specify workspace.

[mrchtr]

We had an offline discussion and decided to move away from a workspace. The cli takes an working_directory argument and uses this as the former base_path