First commit with. Details the organization of the code.

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.