Takes a define xml and returns a dataset with specifications for each variable. The variable will just be the variable, unless the specification for that variable differ between datasets
xml_to_var_spec(doc)define xml document
data frame with variable, length, label columns
Other xml builder:
xml_to_codelist(),
xml_to_derivations(),
xml_to_ds_spec(),
xml_to_ds_vars(),
xml_to_value_spec()