Add support for RepeatDateTime

[gmertes]

Support for RepeatDateTime (#43)

I chose timedelta as the default increment format, to keep a consistent interface with datetime objects. But string formats are also supported just like in RepeatDate.

Supported argument formats:

• star/end: The same as in RepeatDate, plus strings yyyymmddTHHMMSS, yyyymmddTHHMM, yyyymmddTHH
• increment: timedelta object, strings HH:MM:SS, HH:MM

We can think about also supporting int increments to represent hours, but in RepeatDate the int increment is translated to days. This could cause some confusion, so I left it out for now.

I subclassed from RepeatDate because I think the arithmetic functions are the same.

Includes input tests for a variety of format combinations.

[FussyDuck]

All committers have signed the CLA.

[gmertes]

I subclassed from RepeatDate because I think the arithmetic functions are the same.

I just saw in the cpp docs that the arithmetic is in fact not the same as in RepeatDate (the python api docs are incomplete).

So I removed the RepeatDate inheritance. I'm not sure how to implement day/hour/minute etc properties for RepeatDateTime using the ecflow expressions, so I left them out for now.