Decide what date columns are deemed necessary for the data processing pipeline

[JulianBiesheuvel]

For now a hard constraint on FROM and TO DATE in the dataset. But what if only one of the two is available? We have to think about this. For now, if either of the two dates is in invalid or not provided, the data recorded is deleted from the dataset.

[khsjursen]

Right now MBM removes data containing 99 in TO_DATE and FROM_DATE. In the WGMS dataset 99 is used not only for missing day of month (YYYYMM99), but missing month (YYYY9999). For some glaciers this removes a lot of data.

If YEAR is provided then a possibility here is to give the user the option of filling start and end dates of the hydrological year (or winter/summer period). But this is quite a strong assumption such that it should be flagged somehow..