check metadata for storage type

[RobLBaker]

eml/dataset/dataTable/attributeList/attribute/storageType

-Error if empty/missing
-Do anything else with this? More tests?

[RobLBaker]

See comments on issue #61 - the same logic applies.

[RobLBaker]

'test_storate_type()':
-Checks to see if there are the same number of attributes (attributeName) and storageTypes
-Does NOT attempt to verify if the number of storageType elements matches the number of columns in the data package data files (for that functionality, use test_fields_match())
-Does verify that the storageType is valid; i.e. is a member of an accepted list of possible storage types. Currently: string, float, date, factor, or characters. I checked several EML files from EMLassemblyline and ezEML; they only had string, float, or date for storageTypes, but the code for EAL makes it look like factor and character are also valid storageTypes. Valid is based just on ezEML/EAL output: any string in storageType is schema-valid.
-Does NOT attempt to verify that the value in storageType logically matches the type data in the corresponding column.