Simplify parse_facet, set default for textType if not present
Simplify parse_facet, set default for textType if not present

1. D not copy the attrib dict as items need not be popped from it
1. set textType to 'String' as provided in the SDMXML standard. Such efault values should be set at model level rather than by writers.

Caveat: textType is set to String even if the method has been called on an enumeration-type facet.

TODO: Check how to handle enumeration facets properly.

dr-leo/pandaSDMX