Fixes #14 Properly checks dimensional lengths using ndim.

Auxiliary coordinate variables are also allowed, so the dim==name portion has been removed.  Reformatted check to properly check for the increase in dimensionality.
There is a [`_find_boundary_vars` helper method](https://github.com/ioos/compliance-checker/blob/master/compliance_checker/cf/cf.py#L342) this should use instead.