Fix issue #352 and homogenize string concatenation across the code

[jgaffiot]

The first commit modifies only 1 line to fix the issue #352.
The second commit homogenizes the __repr__ method using str.format, and also uses format on some other strings and removes unnecessary concatenation.

[wholmgren]

Looks good, thanks.

Let's add a test to make sure this doesn't happen again. I think the code below can replace test_modelchain.py lines 420-443:

@pytest.mark.parametrize('strategy, strategy_str', [
    ('south_at_latitude_tilt', 'south_at_latitude_tilt'),
    (None, 'None')])  # GitHub issue 352
def test_ModelChain___repr__(system, location, strategy, strategy_str):

    mc = ModelChain(system, location, orientation_strategy=strategy,
                    name='my mc')

    expected = '\n'.join([
        'ModelChain: ',
        '  name: my mc',
        '  orientation_strategy: ' + strategy_str,
        '  clearsky_model: ineichen',
        '  transposition_model: haydavies',
        '  solar_position_method: nrel_numpy',
        '  airmass_model: kastenyoung1989',
        '  dc_model: sapm',
        '  ac_model: snlinverter',
        '  aoi_model: sapm_aoi_loss',
        '  spectral_model: sapm_spectral_loss',
        '  temp_model: sapm_temp',
        '  losses_model: no_extra_losses'
    ])

    assert mc.__repr__() == expected

Please also add a note and your name to the whatsnew file in docs/sphinx/source/whatsnew/v0.4.6.rst

[jgaffiot]

I've updated the pull request with your code and the update of whatsnew/v0.4.6.rst

[wholmgren]

Great, thanks @jgaffiot!