Add installation instructions for librascal in foreword (and clarify scikit-cosmo instructions)

[max-veit]

(This issue was found by Aditi)

The tutorial requires the librascal package, so it would be helpful to have installation instructions in the foreword notebook (the most obvious attempts, pip install librascal and pip install rascal, don't work - in fact, the latter installs a completely different package that just happens to have the same name). The condensed installation instructions for librascal would be:

$ git clone https://github.com/cosmo-epfl/librascal.git
$ cd librascal
$ pip install .

And for installing scikit-cosmo, perhaps also mention that it's called skcosmo on pip?

Thanks!