models. It's a better semantic and improve performance as well as
readability of the code. All the tests passes (except the remark in the
last commit).

Modification of print of models + renaming macro for creation of models in order to be more inclusive.

call of isabsorbing() during the simulation.

Now the package is a bit more efficient than DiffEqJump.jl with the
Catalyst.jl interface which is encouraging, see bench/pkg/catalyst.jl.