Merge date placeholder type forecast initiation time and forecast lead time.

[jfrost-mo]

What problem does your feature request solve?

Some data contains both the forecast initiation time and the forecast lead time in the file name. To allow selecting this kind of data we should be able to have the placeholders in the same date placeholder type.

Describe the solution you'd like

Merge the lead time date type into the forecast initiation date type. There is no conflict, as the lead time uses a different syntax for its placeholders. The documentation will also need updating.

Describe alternatives you've considered