Bug in tutorial with references

[createyourpersonalaccount]

In https://reframe-hpc.readthedocs.io/en/stable/tutorial.html#adding-performance-references there's a definition for myhost:baseline. But as I've discovered after testing, it is more like mysystem:mypartition.

This becomes more confusing since later in the tutorial (see https://reframe-hpc.readthedocs.io/en/stable/tutorial.html#id11) a baseline.py configuration is shown where a hostname myhost is defined for the mytutorialsys system and an environment with name baseline. The correct reference for that would be mytutorialsys:default.

There are three related issues overall with the presentation:

1. The issue I've described above; it is mysystem:mypartition for references and needs to be fixed in the tutorial.
2. The order of presentation should be reversed; we should first discuss configuration files and then references.
3. The valid_prog_environs is not clearly explained. It took me a while to figure out how to request two features, (with valid_prog_environs = ['+A +B'].) In retrospect, it is explained in https://reframe-hpc.readthedocs.io/en/stable/regression_test_api.html#reframe.core.pipeline.RegressionTest.valid_systems but it would be nice if the example showed more than just one feature.

If you agree with these issues I can fix them.