Add of several useful functions for access values of model, lha and
trajectory objects.

Major fix of type unstabilities which improved performances.
For now on, we observe for pygmalion benchmarks:
+2x speed simulation compared to simulation of models.
100x speed in terms of reading values (no allocations).

Fix of a major issue in the simulation: sometimes the number of states
wasn't consistent. Add of test to check the consistency of the
Trajectory structure (unit/check_trajectory_consistency).

Implementation of the lp distance for trajectories.
Add of unit tests for lp distance.

In fine I give the Abstract and Static arrays types up.

- Add of benchmark scripts that compares perf wrt pygmalion.
=> As expected it is bad in terms of reading cost (because of row by row
matrix read) but it is also not good in terms of simulation cost.
hcat seems to perform badly, should investigate.

Files should have the same name as the exported model variable (<:Model)

SIR model.

- Evolution of the ContinuousTimeModel type
- simulate, easy access to Trajectory fields
- fix of several errors

run the tests
Function load_module in core + test