Handling of FREQ in recurrence pattern

[axunonb]

FrequencyType.None

The FrequencyType.None value should be removed because it is neither meaningful in the context of frequency enumeration nor does it have a valid string representation. Its presence could lead to confusion when users create a RecurrencePattern.

In turn RecurrencePattern.Frequency should either become nullable or - for simplicity - have a default value (e.g. FrequencyType.Yearly.

The only test for a valid FrequencyType is necessary in RecurrencePatternSerializer where an invalid input string should throw.

Check for valid RecurrencePattern.Frequency in Evaluator

Currently we have a check for a valid FrequencyType, that originates in the existence of FrequencyType.None.

ical.net/Ical.Net/Evaluation/Evaluator.cs

Lines 46 to 48 in ca0e6e4

	// FIXME: use a more specific exception.
	default:
	throw new Exception("FrequencyType.NONE cannot be evaluated. Please specify a FrequencyType before evaluating the recurrence.");

However, Frequency should always be valid at this stage.
The default case should just trigger a Debug.Fail as a safeguard indicating a missing implementation.
There is no need to throw an Exception for users.

Originally posted by @axunonb in #785 (comment)