Meta code generation for next_state! and simulation of synchronized trajectories. bench/pkg/euclidean_distance*.jl works but segfault with test/automata/euclidean*

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).

edge_candidates on top-level of next_state! function.

allocation per next_state!. Now update_state! functions returns a Symbol
as location instead of modifying a vector.

check_constraints and update_state! functions of an Edge in order to
improve performance.
Acutally the gain is low, but it improved the readabilty of the code.
All tests passed.

integrated poisson model wasn't updated
improvement of plots
fix of synchronized simulation when state is absorbing
All tests passed

All automata were readapted and are in accordance with the last description of the thesis. Tests passed

check_constraint and update_state functions are created inside
create_automaton_*(), by the evaluation of meta_elementary_functions <:
Expr. check_constraint and update_state don't depend on constants::Dict
anymore: constants are interpolated inside the expression.

Add of tests for euclidean distance automaton.
Add a new function euclidean_distance that computes the euclidean
distance of a Trajectory compared to observations along a discrete
timeline.