Psalm (static analysis tool) integration instead of phpstan

[elie29]

Implementing psalm is quite easy.

Required

• [ ] Create a psalm.xml in the project root
• [ ] Adapt the contents from this psalm.xml.dist
• [ ] Run $ composer require --dev vimeo/psalm
• [ ] Run $ vendor/bin/psalm --set-baseline=psalm-baseline.xml
• [ ] Add a composer script static-analysis with the command psalm --shepherd --stats
• [ ] Add a new line to script: in .travis.yml: - if [[ $TEST_COVERAGE == 'true' ]]; then composer static-analysis ; fi
• [ ] Remove phpstan from the project (phpstan.neon.dist, .travis.yml entry, composer.json require-dev and scripts)

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