Fix depreciation warnings that we can fix

[freemansw1]

Resolves #108 and #101. New warnings summary is below. Note that the warnings are either upstream in trackpy or iris-related, which I don't think we want to deal with.

================================================== warnings summary ===================================================
tobac/tests/test_sample_data.py::test_sample_data
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\iris\coords.py:1982: UserWarning: Collapsing a non-contiguous coordinate. Metadata may not be fully descriptive for 'time'.
    warnings.warn(msg.format(self.name()))

tobac/tests/test_sample_data.py::test_sample_data
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\iris\coords.py:1982: UserWarning: Collapsing a non-contiguous coordinate. Metadata may not be fully descriptive for 'projection_y_coordinate'.
    warnings.warn(msg.format(self.name()))

tobac/tests/test_sample_data.py::test_sample_data
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\iris\coords.py:1982: UserWarning: Collapsing a non-contiguous coordinate. Metadata may not be fully descriptive for 'projection_x_coordinate'.
    warnings.warn(msg.format(self.name()))

tobac/tests/test_sample_data.py::test_sample_data
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\iris\coords.py:1976: UserWarning: Collapsing a multi-dimensional coordinate. Metadata may not be fully descriptive for 'latitude'.
    warnings.warn(msg.format(self.name()))

tobac/tests/test_sample_data.py::test_sample_data
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\iris\coords.py:1976: UserWarning: Collapsing a multi-dimensional coordinate. Metadata may not be fully descriptive for 'longitude'.
    warnings.warn(msg.format(self.name()))

tobac/tests/test_sample_data.py::test_tracking_coord_order
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\trackpy\utils.py:201: DeprecationWarning: Importing clear_output from IPython.core.display is deprecated since IPython 7.14, please import from IPython display
    from IPython.core.display import clear_output

tobac/tests/test_sample_data.py::test_tracking_3D
  C:\Users\sfreeman\miniconda3\envs\tobac_dev\lib\site-packages\iris\coords.py:1982: UserWarning: Collapsing a non-contiguous coordinate. Metadata may not be fully descriptive for 'geopotential_height'.
    warnings.warn(msg.format(self.name()))

-- Docs: https://docs.pytest.org/en/stable/warnings.html
============================================ 9 passed, 7 warnings in 7.62s ============================================

[w-k-jones]

Happy for this to be merged