create script to check archive contents

[stevehadd]

As we move to increasing the size of our training dataset, we need to know hether a given event specification (i.e. date range) has the data required in the archive (MASS). There are gaps particularly in the mogreps-g level 1 stage data, so it would be good to be able to quickly check whether all the data exists in the archive to create a PRd dataset for that data range.