Improve variable semantics in test_context.py.

Some variables were named with placeholder text. This commit replaces
those placeholders with meaningful names.
Add test for xbrl.get_linkbase function.
Add tests for xbrl.dup_calcs and xbrl.get_calcs.
Fix line length in thinX.pyw and xbrl.py.

Refactor code so that line lengths do not exceed eighty characters.