Use srun by default in CP2K

[felipeZ]

fix #86

[BvB93]

While you're at it, could you add an else statement after the for / break loop?

If the break statement is never reached then it will get stuck on command = "mpirun", even though mpirun is unavailable in such case. This is guaranteed to crash the job down the line.

for command in ('srun', 'mpirun'):
    try:
        ret = ...
    except OSError:
        break
else:  # In case the previous `break` statement is never reached
    ret = 'sopt'

[felipeZ]

@BvB93 you are write :) I have clean a bit the code and allow the user to invoke her/his own call to MPI using srun or mpirun

[BvB93]

Looking pretty good.
I do feel that, at this point, it might be a good idea to update the method docstring:

"""Run CP2K.

The exact CP2K executable, and whether or not one wants to use ``srun`` or ``mpirun``, can be specified under :code:`self.settings.executable`. 
Currently supported executables are:

* ``"sdbg"``: Serial single core testing and debugging
* ``"sopt"``: Serial general single core usage
* ``"ssmp"``: Parallel (only OpenMP), single node, multi core
* ``"pdbg"``: Parallel (only MPI) multi-node testing and debugging
* ``"popt"`` (default): Parallel (only MPI) general usage, no threads
* ``"psmp"``: parallel (MPI + OpenMP) general usage, threading might improve scalability and memory usage

For example:

.. code-block:: python

    >>> from scm.plams import Cp2kJob

    >>> job = Cp2kJob(...)
    >>> job.settings.executable = "popt"
    >>> job.settings.executable = "ssmp"
    >>> job.settings.executable = "mpirun psmp"

"""