- added tests have a couple of todo's to follow up
- will also need to add:
  - test cases for happy path and other translatable columns
  - the same sort of check for the translatable choices sheet columns
- also added some obvious type annotations and moved repeated usages of
  "default" for the default language to the constants module.

- in the tests in the prior commit it was becoming difficult to discern
  the correct / normal behaviour is for various translation scenarios
- includes numerous todo's for suspect behaviour that may be bugs
- added guidance_hint to translatable columns list

- simplified representation of issue 157 source case where label + hint
  are in French, with a default lang image column, and no default set.
- media columns were not being detected correctly since the survey_sheet
  object nests audio, image and video in a "media" dict.
- updated constants list with comment on other possible translatable
  columns in case pyxform supports them in future.
- general tidy up of test xpaths and usages of them, such as to remove
  combination/shortcut xpaths, specify absence of ref or form attributes
  where relevant, condense guidance/audio/image/video xpaths.
- remove duplicate test todo's for the same issue in a similar context.

- moved TranslationTest.test_missing_media_itext to TestTranslations
- moved new TranslationTest tests to TestTranslationsSurvey
- added TestTranslationsChoices
  - similar to survey tests, added to clarify existing behaviour
  - includes some initial cases, for with/out a lang and choice_filter
  - some overlap with module test_secondary_instance_translations.py
    which should be resolved once these tests are complete.
- added a XPathHelper class to try and make the choices tests more
  legible and easier to document, vs. the constants used for survey
  tests. Could adapt / extend helper for survey tests as well.

- Update TestTranslations and TestTranslationsSurvey that were using
  string constants to use XPathHelper so that the assertions being made
  are easier to follow.
- Add XPathHelper method docstrings to describe what they assert.

- Previously just showing the XForm made it a mystery which of the
  (potentially many) XPath expressions failed to match.

- Add to existing warning for missing translations where the affected
  columns are constraint_message or required_message
- Due to a difference in the internally used name and supported external
  aliases, the TRANSLATABLE_SURVEY_COLUMNS object is now a dict and
  moved to aliases.py so it is closer to the existing aliases dicts to
  help keep them in sync.

- debug=True causes the XForm and other test info to be printed out but
  this shouldn't be present on committed tests.
- If warnings__* parameters are misspelled then they don't do anything.

- checks columns shown in aliases.py
- check done alongside survey check so there's just warning message
  about missing translations
- add choices warning test cases for same set of scenarios as for survey
- update existing survey tests to use warnings__contains or not_contains
  instead of passing in list and doing assertIn / assertNotIn

- the three related functions for this check are parts of a validation,
  and they otherwise clutter up xls2json.py a bit.
- may make sense to move other xls2json checks into the same package
  so it's clearer what validations pyxform does.

- combination of existing tests with same name pattern but in one test
  case that has both warning and no-warning assertions.

- to check whether the missing translation check incurs a reasonable
  time cost, considering that it traverses the full XLSForm dict
  available in xls2json rather than earlier in the pipeline to just
  check sheet headers. It seems there is very little difference for a
  giant form with 500 questions and 2 choices per question.

# Conflicts:
#	pyxform/xls2json.py

- missing translations check can still work while only looking at first
  row, which is a little faster and still passes the tests.
- while profiling, found an inefficient unique check in section.py. The
  benefit is only apparent with forms >1000 questions, but it is
  significant: q=5000 28s vs. 11s, q=10000 103s vs. 21s.

- default_language setting shouldn't influence the warning since the
  underlying issue is missing languages, and the setting just happens
  to be a way around one of the problems that causes.
- updated the warning format to be one line per lang/sheet, with column
  names listed in the line.
- includes corresponding test changes for the above, and re-configures
  how the check func processes data to better match the warning format.