Add properties key to species and reaction parameters

There should be a way to process the property tables of Species or Reaction. For this a key like props or properties has to be provided. See the other AMML objects - some of them already provide such an access.

Example:

water = Species H2O (
    (free_energy: -14.739080832009071, -14.994145508249682) [eV],
    (temperature: 500., 600.) [K]
)
water_props = water.props
free_at_500_K = water.props select free_energy where temperature == 500. [K]
water_props_query = water.props where free_energy < -14.994145508249682 [eV]