GLAH06: Support "Data_1HZ" variables

[trey-stafford]

As of #42 , the code is only setup to read the "Data_40HZ" group from GLAH06 data. There is also a "Data_1HZ" group with the same variables.

The "Data_40HZ" group contains data with a rate of 40Hz. The "Data_1HZ" group contains data with a rate of 1Hz.

The 40Hz data would be the higher resolution data, so this seems like a good default, but ideally a user could choose to access variables from either group.

If the 1Hz data are just a downsampling of the 40Hz data, then this may not actually be necessary. Need to look at the user guide a little more closely!

[trey-stafford]

The user guide (https://nsidc.org/sites/default/files/multi-glah01-v033-v034-userguide_0.pdf) notes (emphasis mine):

GLAS is a profiling instrument that collects data only where the altimeter points, nominally along
the ground track. At 40 pulses per second, the centers of 60 m spots illuminated by the laser on the
earth's surface are separated in the along-track direction by 172 m from a 600 km altitude orbit

So the 40Hz data are the expected sampling rate of the data. Guessing the 1Hz is just a downsampled version to support faster analysis with a smaller sample of data. The user guide doesn't appear to have any details about the 40 vs 1 Hz groups in the data files themselves.

[trey-stafford]

Looking more closely, the 1Hz data also lacks the "Elevation_Surfaces" subgroup that contains elevation data.

Given this and the fact that the 40Hz is a higher sample rate, I think we can close this issue and ignore the 1Hz data group for now.