Add a linter for uniform source formatting

[Gaudeval]

As suggested in the review of #21, we could add a linter to reduce the likelihood of commits with changes lost in formatting.

See https://scalacenter.github.io/scalafix/docs/users/installation.html for an example of tool that could be integrated in the CI, and supports syntactic rules. Not sure if the default ruleset covers spacing, tab width, comment style...