Identify and label tests as unit or functional

Currently, MUSE several labels (`mark`) that identify different kinds of tests and that, when required, let one run them independently of others:

https://github.com/SGIModel/MUSE_OS/blob/f89eb147a6a706fa8f180717f7eefd8336662d0c/pyproject.toml#L96-L101

It will be useful to do the same for unit tests (those that check one what one specific function does) and functional (test a whole functionality or chain of function calls) tests in the same way, so there is more control on how tests are run.