Ignore bounded formula terms.

This PR targets the netCDF CF layer to ensure that formula terms are correctly dealt with when the `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 result.

This PR ensures that the over specified `formula_terms` in the associated bounds variable is ignored.