This commit groups the change operated to the creation of models and
simulate function of a ContinuousTimeModel.
The general idea is to create a concrete type and a simulate function
 per model creation by metaprogramming.
- Now, ContinuousTimeModel is an abstract type. Each creation of a model
defines a concrete type T <: ContinuousTimeModel by meta programming.
- f! and isabsorbing ContinuousTimeModel fields are Symbols.
- simulate(::ContinuousTimeModel) is run by multiple dispatch, according
to the type of the model.

Can't run the whole tests for now but unit/simulate_available_models.jl
runs properly (i've updated the list of models in this commit), and I've
manually checked in the repl that simulations run correctly (distributed
/ plots).

now the package has a better structure for distributed simulations by better definition of functions over workers + test

Fix in simulation of macro generated models. Small changes with variables types of the pkg. Small change in test of macro models.

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.