Subseasonal loading: Load all start dates for obs even if hcst contains some missing data?

@nperez @cdelgado

As discussed offline, this issue is to keep track of a question related to !238 (merged) and #173.

Originally, the initialization date files from the reference dataset were generated using the metadata from the hindcast files retrieved by startR. This meant that if some files from the hindcast were missing, the corresponding dates were set as NA, and therefore the observations corresponding to those missing dates were not loaded.

In !238 (merged), the dates are generated independently from the hindcast file metadata. This means that, in theory, all of the requested period can be loaded for the observations regardless of whether the hindcast has missing data.

The question then become whether it is more correct to always load the entire reference period for the observations, or to match the observations to the hindcast and only load non-missing dates.

The decision for now is to only load the non-missing dates, using the missing files from hcst to "mask" the dates requested for obs, and including a warning to alert the user.

Victòria