Add option for creating a third-body species in the MICM configuration

[mattldawson]

Allow users to specify that a species should be treated as a "third-body" species whose concentration is automatically set to the total air density (mol m-3).

Acceptance Criteria

• A MICM configuration can include a species whose concentration is always equal to the total air density (mol m-3)

Ideas

• Add a new tracer type option for third-body species:

  {
  "name": "M",
  "type": "CHEM_SPEC",
  "tracer type": "THIRD_BODY"
  }

• Have the configuration parser use the micm::Species::ThirdBody() function to create a species flagged with THIRD_BODY

[boulderdaze]

Completed