Make the ACE operator as default for the exchange operator

Description

Following the discussions in #1100, we now make the ACE operator as default for the exchange operator.

Several tests are added. They revealed some problems with the non-ACE case with k-points, which are not fixed in this merge request.

Closes #877 (closed)

News snippet

Make the ACE operator as default for the exchange operator

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.