Add some things Dieter noticed

- change name of test_regexing test method test_sum_by_buckets, since
  the test is really more focused on that and not regexing
- add a few more type conversion tests
- add a link to the Units and Prefixes page on the wiki in the unitconv
  module docstring
call ASTs ASTs, not 'patterns'

I think this hits all the places where the code used "pat" or "pattern"
in places which now refer to trees of AST tuples. Also,
Query.compile_pattern() becomes Query.build_ast().