cylc validate: initial greater than final fail

Closes #1259 

Test added and all pass in my environment.

Validate now checks to see if the initial cycle point is greater than the final cycle point and if so fails with a helpful message. 
* Why capitalise `Initial` and `Final`?
* It will be useful to include the cycle point strings in the error message.
* Can we say that a date in the future is *greater than* a date in the past?
* This continuation, I believe, does not conform to PEP8.