Add companion variables for Flow-dep cross-timescale POD to fieldlists

[wrongkindofdoctor]

The GFDL and CMIP field lists require the following variables to support the Flow-Dependent Cross-Timescale Diagnostic POD
(see PR #127 discussion):

"Z250": {
      "standard_name": "geopotential_height_250_mbar",
      "units": "m",
      "ndim": 3
    },
    "T250": {
      "standard_name": "temperature_250_mbar",
      "units": "K",
      "ndim": 3
    },
    "T500": {
      "standard_name": "temperature_500_mbar",
      "units": "K",
      "ndim": 3
    }

[wrongkindofdoctor]

It turns out that the geopotential height and temperature entries in the current field lists will support variables requested at specific levels, provided that the settings file entries for z250, t250, and 500 define a static variable attribute, e.g.,:

"scalar_coordinates": {"lev": 250}