The dictionary keys are currently never actually used directly in the
package. Loops go over the `k, v` of the dictionary `format1` however
references to field names are only ever made through the `.name`
attribute of `Spec` objects.

Also fix docstrings in `Spec` classes to reference the correct format
field.

The `alternative-names` test found a mistype of `type` in the field
definition of the `alternative-names` spec that should have been
`dtype`.