Resolve "rename poisson_libisf_oct to poisson_psolver_oct"

Description

This MR is one task related to #277, aiming at updating the support of the PSOLVER library. This MR does not yet rename HAVE_LIBISF to HAVE_PSOLVER_1_7, in order not to break the tests for other branches (due to the required change of the reference config.h).

The renaming will be done when also the new PSOLVER_1_8 toolchain will be added to the buildbot, as that also will required a change in the reference.

News snippet

This MR is one task related to #277, aiming at updating the support of the PSOLVER library

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.

Closes #278 (closed)