Add inherit option to tasks to inherit from other tasks (e.g., root)

[GeigerJ2]

Currently, the root task that is supposed to contain runtime information to be inherited by all other tasks (e.g. HPC host and account) is defined in the same way the other tasks. This might lead to some complications down the road, as it should actually be treated entirely differently from the other tasks. Therefore, I propose to instead add it as a separate section under runtime, for instance called runtime_defaults. This allows for it to be parsed separately from the other tasks (which are actual tasks to be executed, while root is not), and populate the other tasks accordingly.

To keep things simple while developing, I just commented it out for now.

[leclairm]

There are pros and cons but it's not urgent anyhow. Having a special rule for root doesn't seem too cumbersome to me but it's fine to keep it commented out for now.

[GeigerJ2]

Possibly, instead, add an inherit option in the YAML file for tasks, to inherit from arbitrary tasks, not necessarily root.