'merge_datasets_by_datenum/table' doesn't always work as advertised

[gremau]

When creating a new fluxall file in which the early year data is missing (such as GLand 2007), 'merge_datasets_by_datenum' does not properly pass the start date (first of the year) on to 'dataset_fill_timestamp'.

This does not occur for some reason when there is an existing fluxall file.