Update interpolate.py

fix docstring of `nearest_neighbour_data_value`
Update trajectory.py

fix docstring format in `interpolate` method
Added support for variable resolution fieldsfile loading.
Merge pull request #708 from andreas-h/patch-1

Updated a docstring in interpolate.
Merge pull request #710 from andreas-h/patch-2

Update docstring in trajectory.
Resolved circular imports in Iris.
Merge pull request #619 from marqh/varff

Added variable resolution fieldsfile loading.
Calculates correlation between cubes