Workaround for a problem with ifort 2018.1 and OpenMP.

See merge request octopus-code/octopus!637

Merge branch '211-move-the-poisson-solver-to-the-system-as-we-need-to-have-one-instance-of-the-solver-per-system' into 'develop'

Resolve "Move the poisson solver to the system, as we need to have one instance of the solver per system"

Closes #211

See merge request octopus-code/octopus!624

Enable OpenMP in isf Poisson solver (SGFFT) to increase performance

See merge request octopus-code/octopus!633

The Ewald long-range for 2D was leading to NaN for energy or forces for thick slabs.

See merge request octopus-code/octopus!632

The fft solver is already parallelized using openmp, but it has not been
used since 2011. Now it is enabled and makes the OpenMP version faster.

Bugfix: td_occup with states parallelization and isolated systems

See merge request octopus-code/octopus!629

Hotfix unoccc output

See merge request octopus-code/octopus!628

Fix various problems related to the header of td_occup output for isolated systems in states parallelization.

The variable OutputDuringSCF was not used for unocc calculations, unlike what the documentation was saying.

In case the option TDFreezeU was used, the potential was not updated just after the restart for the correct (GS) value of the U, causing the first iteraction after the restart to used wrongly the U from the orbitals instead of the GS one.

Resolve "Add an input block to define the systems to run"

Closes #191

See merge request octopus-code/octopus!613

Disable multisystem test, which is producing a memory leak in the Poisson solver. This needs to be fixed before we activate the test.