Nimrod xy names

When loading NIMROD data defined on an OSGB grid the resulting coordinates are named 'x' and 'y'. This PR changes the names to 'projection_x_coordinate' and 'projection_y_coordinate' so that ```iris.plot``` and ```iris.quickplot``` automatically identify the cube as suitable for plotting on a map. This will mean that code like:
```
import matplotlib.pyplot as plt

import iris
import iris.quickplot as qplt

filename = 'mynimroddata'
cube = iris.load_cube(filename)
qplt.pcolormesh(cube)
plt.gca().coastlines(resolution='10m')
```
works. Currently, it complains: AttributeError: 'AxesSubplot' object has no attribute 'coastlines'.