Add option for particle type level default output behaviour to new output list scheme

This small update adds the possibility to define a default output policy for each particle type. This is done through the (fake) field 'Standard' in the output selection YAML file (e.g. Standard_DM: off). This is exactly equivalent to switching off each individual particle field for this type, but (a) involves less work, and (b) is robust to the addition of new output fields after an output selection YAML file has been created.

@jborrow -- Could you please also have a look at this, and check whether it looks ok?

Implements #680 (closed).