Support species with constant concentrations

[DanielKneipp]

Change the react() function to make it support species with constant concentrations.

For example, a marker as the first character of the species name (#) could mean that this species has a constant concentration.

react(
    species   = c('#A', 'B', 'C'),
    ci        = c(1e3, 1e3, 0),
    reactions = c('#A + B -> C'),
    ki        = c(1e-7),
    t         = seq(0, 72000, 10)
)