A reactant/product pool for reaction scheme calculations. The pool contains its initial concentration in Units defaulting to mM but which may be any others consistent with those used in the RScalc object which defines the rest of the environment.
The semi-private fields posx and posy are the position of the pool, for drawing purposes only.
|     field |     type   |     contents / default value |     |
| C_0 | double | 1.0 | C_0: initial concentration |
| D_diff | double | 0.0 | D_diff: diffusion constant. Only relevant for models involving spatial concentration variation |
| buffering | choice | buffering: the type of buffering or other control of concentration. One of: none | |
| conc. profile | list selection; boolean; one only | Profile |