add model data availability check

[leiyangleon]

Probably we should also have a check in the code when models are trying to be downloaded to a file that does not exist, it provides a warning and TBD if we just eliminate the epic and continue with those that exist or if we error out altogether whcih goes into the loggers.

Right now, the worst-case lag time is selected to guarantee all the data files satisfying the lag time check exist. So it is not urgent to add this data availability check, but it could be included to make it more robust in case something unexpected happens.

[dbekaert]

Currently some of the models which have a lag are printed to the logger and screen to be available to present:

Weather model MERRA2 is available from 1980-01-01 00:00:00-Present