DoubleML 0.4.0

• Release highlight: Clustered standard errors for double machine learning models #119
• Apply styler as described in the wiki (https://github.com/DoubleML/doubleml-for-r/wiki/Style-Guidelines) and add a corresponding CI on github actions #120 #122
• Other refactoring, bug fixes and documentation updates #127 #129 #130 #131 #132 #133