Additional Cell Boundary checks for CF-1.7

[RosalynHatcher]

Moved from #32

7.1 Cell Boundaries
Requirements:

• Starting with version 1.7, a boundary variable must have a formula_terms attribute when it contains bounds for a parametric vertical coordinate variable that has a formula_terms attribute. In this case the same terms and named variables must appear in both except for terms that depend on the vertical dimension. For such terms the variable name appearing in the boundary variable’s formula_terms attribute must differ from that found in the formula_terms attribute of the coordinate variable itself. The boundary variable of the formula_terms variable must have the same dimensions as the formula_terms variable, plus a trailing dimension (CDL order) for the maximum number of vertices in a cell, which must be the same as the trailing dimension of the boundary variable of the parametric vertical coordinate variable. If a named variable in the formula_terms attribute of the vertical coordinate variable depends on the vertical dimension and is a coordinate, scalar coordinate or auxiliary coordinate variable then its bounds attribute must be consistent with the equivalent term in formula_terms attribute of the boundary variable.