Fixes to allow reading sea surface height on dataset with 3D sigma

Here are changes I made to get Iris to read the sea surface height variable from this model with 3D sigma:
```
url='http://fvcom.smast.umassd.edu:8080/thredds/dodsC/FVCOM/NECOFS/Forecasts/NECOFS_GOM3_FORECAST.nc'
zeta = iris.load_cube(url,'zeta')
```
1. I had to first remove the check on whether variables are used in the `formula_terms`, since `zeta` the sea surface height variable, is used in the sigma coordinate computation.

2. I had to remove the check on expected dimensions, as Iris was expecting only 1D for sigma, but this model has 2D (sigma varies with every node in this unstructured grid ocean model).   BTW, it seemed a bit strange that this prevented me from reading 'zeta', since it doesn't have a vertical coordinate.

I realize this might have consequences for other applications.  If it does, I hope you find a solution that will allow me to read this dataset and not cause other problems.    The full application that I'm using this for is here, a tools to automatically compare water levels from various models to data:
http://nbviewer.ipython.org/github/rsignell-usgs/notebook/blob/master/CSW/IOOS_inundation.ipynb
I'm using Iris to give back the lon,lat and time coordinates of these models.

Thanks,
Rich