Export to static IDS nodes

[SBlokhuizen]

Allow exporting to non-time-dependent 0D IDS nodes. These waveforms may only consist of a single constant tendency, e.g.

equilibrium/vacuum_toroidal_field/r0:
  - {type: constant, value: 6.2}
equilibrium/code/name:
  - {type: constant, value: "NICE"}
equilibrium/ids_properties/homogeneous_time:
  - {type: constant, value: 0}

[maarten-ic]

Could we reduce complexity by introducing a ConstantWaveform subclass (see shorthand notation) and require that instead of checking for a single constant tendency in the export?