Make old 'cycling = Yearly' sections explicit.

We don't currently have backward compatibility for pre cylc-6 suites with this style of cycling declaration:
```
[scheduling]
   cycling = Yearly
   [[dependencies]]
      [[[2014,2]]]
         graph = ...
```
The ```cycling = Yearly``` item provides a default for the dependency sub-sections, so the sub-section above is equivalent to ```Yearly(2014,2)```.  I know this form is used by some people.

This change detects this usage and converts to the explicit ```Yearly(2014,2)``` form, which is already backward compatible.

@benfitzpatrick - please review (this is relevant to your #1131).