Ignore bounded formula terms.

This PR targets the netCDF CF layer to ensure that formula terms are correctly dealt with when a `formula_terms` attribute is also present on the bounded variable of a dimensionless vertical coordinate.

Previously, if a dimensionless vertical coordinate (i.e. it has a `formula_terms` attribute) was bounded and the associated bounds variable also had a `formula_terms` attribute, then a riot would/could ensue.

This PR ensures that the over specified `formula_terms` attribute in the associated bounds variable is ignored. It also ensures that the dimensionality of the formula term variable being added to the `cf_group` of the data variable fully spans the dimensionality of the data variable.