Allow conditional masking

Ideally, points in parameter space could be masked conditionally depending on values of other parameter dimensions.

Syntax suggestion:

parameter_space:
  x: !pdim
    default: 0
    range: [-10, 11]
  y: !pdim
    default: 0
    range: [-10, 11]
  z:
    default: 0
    range: [100]
    rules:          # sequence of OR-connected conditions
      positive_x:   # arbitrary name for condition
        - x: [>=, 0]
        # ... more AND connected conditions here
      positive_y:
        - y: [>=, 0]
        # ...
      # ... more OR connected conditions here

Remarks:

• arbitrary combinations of conditions should be possible
• should be possible to invert the selection easily, e.g. via rules_options
• ...