Add .gitignore and remove wrong tracked directories

[PiroX4256]

Please add .idea and target to gitignore and remove them by using git rm --cached.

Check these slides for more information: https://webeep.polimi.it/pluginfile.php/376496/mod_folder/content/0/slides-gitignore-struttura-repository_rev01.pdf?forcedownload=1

Sample gitignore for Java and Intellij: https://www.toptal.com/developers/gitignore/api/java,intellij

[Giovanni-Mattia-Codemo]

Added .gitignore to the repo

[cerutigiacomo]

Remove also the .iml file

[Giovanni-Mattia-Codemo]

.iml file removed