Improve differential operators (#528)

* Improve grad logic and fix issues

* Add operators' fast versions

* Fix bug in laplacian + new tests + restructuring

Co-authored-by: Dario Coscia <dariocos99@gmail.com>

* fix advection bug

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Co-authored-by: Dario Coscia <dariocos99@gmail.com>

Author: GiovanniCanali