add "add-model" command, which adds model properties to the first

add "add-model" command, which adds model properties to the first
free /models/model[*] slot and lets the model manager load the
model. The target address is returned under "property". Simple
use in Nasal:

  fgcommand("add-model", props.Node.new({
          "path": "Models/Fauna/cow.ac",
          "latitude-deg": 16.234,
          "longitude-deg": 48.321,
          "elevation-ft": 300,
  });

An "elevation-m" property can alternatively be used and overrides
the "elevation-ft" property.

Each of the properties "latitude-deg", "longitude-deg", "elevation-ft",
"heading-deg", "pitch-deg", and "roll-deg" can alternatively be used
with "-prop" suffix. Their values must then be property path strings
pointing to a node that can be changed at runtime to move the model.