Update install_and_test.md

# If you need to put hands on the source code

In case you find that you need to edit the source code of hotwheels (maybe you found a bug), you

can re-install a python module in editable mode. For instance, let's suppose that now you need to edit the timestep module and re-run all tests. You can do the following 

can re-install a python module in **editable mode (use -e in pip install)**. For instance, let's suppose that now you need to edit the timestep module and re-run all tests. You can do the following 

```bash

pip install git+ssh://git@git.ia2.inaf.it/hotwheels/timestep.git@v0.0.0alpha 

git clone ssh://git@git.ia2.inaf.it/hotwheels/timestep.git

cd timestep

# very IMPORTANT the -e will install in editable mode

# so you can change the module code and effects will take place