Fix the implementation of recurrence format number 1

[benfitzpatrick]

The first recurrence format does not define repetitions within the given start and end date - it actually just extracts the interval between the start and end date and repeats that starting at the start date. We need to alter the code to represent this.

[MetRonnie]

The current incorrect implementation also leads to a TypeError if the repetitions arg is omitted when creating a TimeRecurrence instance of format 1, e.g.

TimeRecurrence(start_point=TimePoint(year=2020), end_point=TimePoint(year=2022))

[MetRonnie]

To give an example of and clarify how our implementation of recurrence fmt 1 is wrong, take the following TimeRecurrence:

R4/2004-01-01/2004-01-02

According to ISO 8601 this should be a recurrence starting at 2004-01-01T00, with an interval duration of 1 day (the difference between the two date-times given) and so ends at 2004-01-04. This is functionally equivalent to

R4/2004-01-01/P1D

However, in our incorrect (but unfortunately longstanding) implementation, the example above means a recurrence starting at the same date-time but ending at 2004-01-02T00 with an interval duration of 6 hours.

[MetRonnie]

Quote from ISO/WD 8601-1 (ISO/TC 154/WG 5 N0038, 2016)

4.5.1 Means of specifying recurring time intervals

A recurring time interval shall be expressed in one of the following ways.

• a) By a number of recurrences (optional), a start and an end. This represents a recurring time interval of which the first time interval is identified by the last two components of the expression and the number of recurrences by the first component. If the number of recurrences is absent, the number of occurrences is unbounded.
• b) [Not implemented in metomi-isodatetime]
• c) By a number of recurrences (optional), a start and a duration. This represents a recurring time interval of which the first time interval is identified by the last two components of the expression and the number of recurrences by the first component. If the number of recurrences is absent, the number of occurrences is unbounded.
• d) By a number of recurrences (optional), a duration and an end. This represents a recurring time interval of which the last time interval is identified by the last two components of the expression and the number of recurrences by the first component. If the number of recurrences is absent, the number of occurrences is unbounded.

(emphasis mine)

[MetRonnie]

Another thing: the current implementation suffers from floating point errors due to dividing the difference between start_time and end_time by the number of repetitions to get the interval duration. This makes the comparison operations tricky