0.1.1-2021.04

• Allow additional properties for header section of schema.
• Add version tag to schema
• Correct 'Uncertainty' property spelling under Calibration Uncertainty.
• Add solar to the list of plant types.
• Measurement types, add:
  • air_density
  • resistance
  • power
  • energy
  • albedo
• Measurement Units, add:
  • kg/m^2
  • A
  • W, kW, MW
  • kWh, MWh
• Sensor types, add:
  • albedometer
  • solar_compass
• Add Zenodo webhook