add more checks to visualisation.vector_plot and test checks

[nhsavage]

At the moment there are a number of issues with the vector_plot function

1. it checks whether there is a single X and Y coord but then doesn't raise an exception
   2, it doesn't check whether we have u and v winds (the names of these will be different depending on whether the data is on a rotated pole or not and whether they have been rotated already). At the moment, a user could pass U and V the wrong way around or even pass temperature and pressure and no error would be raised.