Add support for `$atan`, `$atan2` and `$atanh` aggregation operators

[christophstrobl]

Available since MongoDB 4.2.

• https://docs.mongodb.com/manual/reference/operator/aggregation/atan/
• https://docs.mongodb.com/manual/reference/operator/aggregation/atan2/
• https://docs.mongodb.com/manual/reference/operator/aggregation/atanh/

[petitcl]

Hi ! I am interested in handling this issue ! Is it ok for you ?

[mp911de]

@petitcl there was already a pull request #3794 submitted by the time of your comment.