catch setting of coordiantes and bounds for netcdf saver (string typing)

[marqh]

fix to make #2158 functional for newer netcdf4-python

i am not convinced this catch all potential cases, only the ones we test

a few words prior to merge would be helpful

[pelson]

Does cf_var_grid.grid_mapping_name need to be considered?

[pelson]

Merge at will. 👍

[marqh]

the mpl pin should not be changed in this PR

[pelson]

Python3 issue with your unit test:

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FAIL: test_spherical_geog_cs (iris.tests.unit.fileformats.netcdf.test_Saver.Test__create_cf_grid_mapping)
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Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test-environment/lib/python3.4/site-packages/Iris-1.11.0.dev0-py3.4-linux-x86_64.egg/iris/tests/unit/fileformats/netcdf/test_Saver.py", line 570, in test_spherical_geog_cs
    self._test(coord_system, expected)
  File "/home/travis/miniconda/envs/test-environment/lib/python3.4/site-packages/Iris-1.11.0.dev0-py3.4-linux-x86_64.egg/iris/tests/unit/fileformats/netcdf/test_Saver.py", line 546, in _test
    actual = self._variable_attributes(coord_system)
  File "/home/travis/miniconda/envs/test-environment/lib/python3.4/site-packages/Iris-1.11.0.dev0-py3.4-linux-x86_64.egg/iris/tests/unit/fileformats/netcdf/test_Saver.py", line 537, in _variable_attributes
    mock_grid_variable = self._grid_mapping_variable(coord_system)
  File "/home/travis/miniconda/envs/test-environment/lib/python3.4/site-packages/Iris-1.11.0.dev0-py3.4-linux-x86_64.egg/iris/tests/unit/fileformats/netcdf/test_Saver.py", line 528, in _grid_mapping_variable
    grid_variable.grid_mapping_name)
AssertionError: b'latitude_longitude' != 'latitude_longitude'
----------------------------------------------------------------------

[pelson]

👍