user labelling for non-scalar values that do not cooperate with xarray

Issue was initially opened by Julian Weninger

See also: utopia/utopia#58

For non-scalar values that do not cooperate with Xarray we could use user defined labels.

e.g.

initial_density: !sweep
  default: [0.1, 0.9]
  values: ['state_0': [0.1, 0.9], 'state_1': [0.9, 0.1]]