Parameters
    ----------
    dataframe: pandas.DataFrame
        dataframe that contains the data
    settings: dict
        with required settings, see below
    keep_dir: path
        if not None, copy modeling dir here


    settings = {
        'rho': 100,  # resistivity to use for homogeneous model, [Ohm m]
        'elem'
        'elec'
        '2D' : True|False
        'sink_node': '100',
    }

def compute_K(dataframe, settings, keep_dir=False):
    """
    Parameters
    ----------
    dataframe: pandas.DataFrame
        dataframe that contains the data
    settings: dict
        with required settings, see below
    keep_dir: path
        if not None, copy modeling dir here


    settings = {
        'rho': 100,  # resistivity to use for homogeneous model, [Ohm m]
        'elem'
        'elec'
        '2D' : True|False
        'sink_node': '100',
    }

    """
    if settings is None:
        print('using default settings')
        settings = get_default_settings()

    if not os.path.isfile(settings['elem']):
        raise IOError(
            'elem file not found: {0}'.format(settings['elem'])
        )

    if not os.path.isfile(settings['elec']):
        raise IOError(
            'elec file not found: {0}'.format(settings['elec'])
        )

    # read grid file and determine nr of cells
    with open(settings['elem'], 'r') as fid:
        fid.readline()
        cell_type, cell_number, edge_number = np.fromstring(
            fid.readline().strip(),
            sep=' ',
            dtype=int,
        )

    # generate forward model as a string
    forward_model = '{0}\n'.format(cell_number)
    forward_model += '{0} {1}\n'.format(settings['rho'], 0) * cell_number

    full_path_elem = os.path.abspath(settings['elem'])
    full_path_elec = os.path.abspath(settings['elec'])

    pwd = os.getcwd()
    with tempfile.TemporaryDirectory() as invdir:
        os.chdir(invdir)
        # create tomodir directory structure
        for dir in [
            'exe',
            'mod',
            'config',
            'inv',
            'grid',
            'rho',
        ]:
            os.makedirs(dir)

        # save forward model
        with open('rho/rho.dat', 'w') as fid:
            fid.write(forward_model)

        shutil.copy(full_path_elem, 'grid/elem.dat')
        shutil.copy(full_path_elec, 'grid/elec.dat')

        print('SETTINGS')
        print(settings)

        cfg = CRcfg.crmod_config()
        if settings.get('2D', False):
            # activate 2D mode
            print('2D modeling')
            cfg['2D'] = '0'
            cfg['fictitious_sink'] = 'T'
            cfg['sink_node'] = settings.get('sink_node')
        else:
            cfg['2D'] = 1

        cfg.write_to_file('exe/crmod.cfg')
        subprocess.call('cat exe/crmod.cfg', shell=True)

        config_orig = _write_config_file('config/config.dat', dataframe)

        os.chdir('exe')
        binary = CRbinaries.get('CRMod')
        subprocess.call(binary, shell=True)
        os.chdir('..')

        # read in results
        modeled_resistances = np.loadtxt(
            'mod/volt.dat',
            skiprows=1,
        )

        # now we have to make sure CRMod didn't change the signs
        changed_sign = (config_orig[:, 1] == modeled_resistances[:, 1])
        modeled_resistances[~changed_sign, 2] *= -1

        if settings.get('norm_factor', None) is not None:
            modeled_resistances[:, 2] /= settings.get('norm_factor')

        K = settings['rho'] / modeled_resistances[:, 2]
        if isinstance(dataframe, pd.DataFrame):
            dataframe['k'] = K
        if keep_dir is not None and not os.path.isdir(keep_dir):
            shutil.copytree('.', keep_dir)
            print('Copy of modeling dir stored here: {}'.format(keep_dir))

    os.chdir(pwd)
    return K