Defining data chunks with data downlink times

[altuson]

When defining data chunks with data downlink times in _data_chunks(), we use the file stored here. We then check that there are 2, 4 or 8 data chunks, depending upon the sector. There are 8 data chunks for sectors 97/98 which were the first four-orbit sectors. In the future, we will need to add additional checks if there are more sectors which break from the trend. Ideally we will come up with an automated way to check this, but we at least need to manually update when necessary.