Fix Traceback with periodicity in DashboardView

[xispa]

Description of the issue/feature this PR addresses

Traceback in Dashboard view

Current behavior before PR

Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.lims.browser.dashboard.dashboard, line 184, in __call__
  Module bika.lims.browser.dashboard.dashboard, line 208, in check_dashboard_cookie
  Module bika.lims.browser.dashboard.dashboard, line 248, in _create_raw_data
  Module bika.lims.browser.dashboard.dashboard, line 323, in get_sections
  Module bika.lims.browser.dashboard.dashboard, line 588, in get_analyses_section
  Module bika.lims.browser.dashboard.dashboard, line 786, in fill_dates_evo
AttributeError: 'DashboardView' object has no attribute 'periodicity‘

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I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.