Use new calculator in favor of set

This MR introduces the .new(**kwargs) function to the GPAW calculator, that creates a new instance of the calculator with identical input parameters, except those supplied as keyword arguments **kwargs. The .new() function can in many places be used instead of the .set() function, that changes the parameters in the existing calculator, and is the source of potential bugs.

In this MR, the .set() function is replaced by .new() in several tests.

Solves part of #705