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).

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).

- a script to run the different categories
- now the execution of simulation/ scripts is checked
- several minor changes and add of tests

New directory : models/_bench_perf_tests.  Models for performance benchmarking purposes.

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

Two folders:
- core/ which contains the essential files for the packages
- tests/ which contains tests and benchmarks of other packages / methods

For now on:
- we wrote minimal tests in tests/ (a simulation of SIR model) that should run for
a first minimal version of the package
- we described in core/ the general structure of essential
types, how they articulate each others and the minimum methods we should
implement for a first version that runs the tests.