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.

can be set by the user. Changes of SIR and ER.

- 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.

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.