Tests: replace namespace with lxml import

Besides swapping imports, also rewrite tests for unit utilities to
expect an lxml element.
Remove namespace helper functions and tests
xbrl.py: remove duplicated functions
xbrl.py: rewrite get_file_namespace() to use lxml
Rewrite xbrl.get_linkbase() to use lxml
Rewrite xbrl.calc_values to use lxml
Rewrite xbrl.add_namespace() to use lxml
Remove namespace import from xbrl.py
Move thinX.pyw to lxml
Rewrite units() to use new lxml xbrl.py functions