missing value incorrectly allowed in examples of time coordinate

[taylor13]

The NUG forbids missing values in coordinate variables. Therefore in examples H.4 and H.5 the missing_value attribute should not appear attached to time(time). [It is, however, sometimes o.k. to permit missing values for time in "unused elements in discrete sampling geometries" (when time doesn't appear as a formal coordinate variable), as illustrated in examples H.3, H.16, and H.18, H.20, and H.21, so these examples are o.k. as is.]