Raise a clear error converting a truncated TimePoint to a date

[gaoflow]

Fixes #265.

Converting a truncated TimePoint to a calendar or ordinal date raised an obscure TypeError instead of a clear error:

from metomi.isodatetime.data import TimePoint
a = TimePoint(year=1, week_of_year=2, truncated=True,
              truncated_property='year_of_decade')
a.to_calendar_date()
# TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

Cause

to_calendar_date() / to_ordinal_date() delegate to get_calendar_date() / get_ordinal_date(). For a truncated week-date point these reach get_calendar_date_from_week_date(year, week_of_year, day_of_week) with day_of_week unset, so (week_of_year - 1) * 7 + day_of_week - 1 evaluates int + None and raises the unhelpful TypeError. A truncated TimePoint has no fully-determined year or day, so it fundamentally cannot be converted to an absolute calendar or ordinal date.

Fix

Guard get_calendar_date() and get_ordinal_date(): when the point is truncated, raise a clear ValueError, mirroring the existing truncated-comparison guard ("Cannot compare truncated to non-truncated TimePoint: ..."). The to_calendar_date() / to_ordinal_date() wrappers surface this directly, and non-truncated conversions are unchanged.

Added a regression test (test_timepoint_truncated_to_calendar_ordinal_date) covering both methods, and added my name to the Code Contributors list per CONTRIBUTING.md.

Disclosure: I prepared this fix with AI assistance under my direction; I reviewed and verified the change and the test myself.