20111229
========

- Wrote the migration handler 
  :func:`lino.apps.dsbe.migrate.migrate_from_1_3_2` 
  which sets the new `build_time` fields on existing data.
  
- Added a new test "Overlapping contracts" 
  to :meth:`lino.apps.dsbe.models.Person.data_control`.
  On result is that the preview takes now quite some time 
  and should be cached.
  

One surprise while working on the migration handler::

  Problem installing fixture '/usr/local/django/dsbe_eupen/fixtures/d20111229.py': Traceback (most recent call last):
    ...
    File "/usr/local/django/dsbe_eupen/fixtures/d20111229.py", line 266, in lino_datacontrollisting_objects
      yield create_lino_datacontrollisting(1,True,None)
    File "/var/snapshots/lino/lino/apps/dsbe/migrate.py", line 908, in create_lino_datacontrollisting
      obj.build_time = obj.get_cache_mtime()
    File "/var/snapshots/lino/lino/mixins/printable.py", line 705, in get_cache_mtime
      filename = self.get_cache_filename()
    File "/var/snapshots/lino/lino/mixins/printable.py", line 701, in get_cache_filename
      bm = get_build_method(self)
    File "/var/snapshots/lino/lino/mixins/printable.py", line 453, in get_build_method
      bmname = elem.get_build_method()
    File "/var/snapshots/lino/lino/mixins/printable.py", line 694, in get_build_method
      return settings.LINO.config.default_build_method
  AttributeError: 'Lino' object has no attribute 'config'

Explanation: the dpy fixtures are of course executed before any the kernel 
setup has been done. That's why there was no `config` object jetzt in the :class:`lino.Lino` 
instance. 
And the site's :attr:`lino.models.SiteConfig.default_build_method` has not yet been restored.

Solution: new configuration parameter :attr:`lino.Lino.site_config_defaults`.
And instead of setting a `config` instance variable during kernel setup, we now have 
a property `site_config` which lazily calls `get_site_config`.