Get available dates in Input made more robust

Input::getDates did not work properly when data files are organized
within date and/or init folders.
Fixes unsafe use of vector of members in Input

Use getMembers(), instead of mMembers
Fixes SelectorAnalog for non-zero inits and by adding 'dataset'

* Add 'dataset' option, to specify which dataset to use for analogs.
  Don't rely on the default dataset because the downscaling isn't
  working within Data
* Fixes problem when running analog method with non-zero inits