bug: Alarm of event with date start creates error

[niccokunzmann]

Describe the bug

When I generate all alarms of the file 173-only-modifications-error.ics, the recurring-ical-events generates an error because it tries to set the trigger to a date.

We should

• check that we can calculate alarms for dates
• that having a timedelta from a date still correctly calculates the time of the alarm

To Reproduce

ICS file

Expected behavior

I would always like to get all alarms without problem.

Console output

Version:

Additional context

mergecal/icalendar-scheduling-benchmark#6

Suggested implementation

• add an ICS file, example:
  

  python-recurring-ical-events/test/calendars/issue-15-duplicated-events.ics

  Line 1 in f4a90b2

  BEGIN:VCALENDAR

• add a test, example:
  

  python-recurring-ical-events/test/test_issue_15.py

  Line 21 in f4a90b2

  def test_sequence_is_not_present(calendars):

• fix the bug and ensure the test code passes