Create controlled ATL03 HDF5 test file

[asteiker]

To better control these performance tests, we ought to create stripped down ATL03 files based on information from #23 and #26. The goal is to better control for the complexities in the native data. We can then reduce the noise as we investigate switches in the HDF5 library to increase performance in the cloud.

### Tasks
- [x] Determine key variables, including quality flags, needed for higher level products.

[asteiker]

@andypbarrett to provide key variables, including quality flags, needed for higher level products. Utilize ATBD and earthdata-varinfo to inform what information is needed for a controlled ATL03 HDF5 test file.

[andypbarrett]

I've looked at ATL07 and ATL06. Below are a list of candidate variables. I've left out geophysical corrections that are already included in _hph. Paths are from ATL03 data dictionary. Variable lists are from table 2 in ATL07 ATBD and table 5-1 in ATL06 ATBD.

11 out of the 13 variables in /gtx/heights are used by one or both of ATL07 and ATL06, and 12 out of 37 variables from /gtx/geolocation are used.

ATL03 Variable	Description	Data type	Data Rate	Source
_/gtx/heights/deltatime	Elapsed GPS seconds	Double(:)	Photon Detection Rate	6 + 7
_/gtx/heights/dist_phacross	Distance off RGT	Float(:)	Photon Detection Rate	6 + 7
_/gtx/heights/dist_phalong	Distance from equator crossing	Float(:)	Photon Detection Rate	6 + 7
_/gtx/heights/hph	Photon WGS84 height	Float(:)	Photon Detection Rate	6 + 7
_/gtx/heights/latph	Latitude	Double(:)	Photon Detection Rate	6 + 7
_/gtx/heights/lonph	Longitude	Double(:)	Photon Detection Rate	6 + 7
_/gtx/heights/pce_mframecnt	PCE major frame counter	UINT(:)	Photon Detection Rate	6
_/gtx/heights/ph_idchannel	Receive channel id	UINT(:)	Photon Detection Rate	6
_/gtx/heights/ph_idpulse	laser pulse counter id	UINT(:)	Photon Detection Rate	6
_/gtx/heights/qualityph	Photon quality	INT	Photon Detection Rate	7
_/gtx/heights/signal_confph	Photon signal confidence	INT	Photon Detection Rate	6
_/gtx/geolocation/deltatime	Elapsed GPS seconds	DOUBLE(:)	Segment rate	6
_/gtx/geolocation/ph_indexbeg	Index of first segment photon	INT	Photon Detection Rate	6
_/gtx/geolocation/podppdflag	Geolocation quality	INT	Photon Detection Rate	6
_/gtx/geolocation/segment_distx	Segment distance from equator	Double(:)	Segment rate	6
_/gtx/geolocation/segmentid	along-track segment id	INT(:)	Segment rate	6 + 7
_/gtx/geolocation/segmentlength	along-track segment length	Double(:)	Segment rate	6 + 7
_/gtx/geolocation/segment_phcnt	Number of photons	INT(:)	Segment rate	6
_/gtx/geolocation/sigmaacross	across-track geolocation accuracy	Float(:)	Segment rate	6 + 7
_/gtx/geolocation/sigmaalong	along-track geolocation accuracy	Float(:)	Segment rate	6 + 7
_/gtx/geolocation/sigmah	height uncertainty	(Float(:)	Segment rate	6
_/gtx/geolocation/surftype	Surface type	INT(:, 5)	Segment rate	7
_/gtx/geolocation/velocitysc	spacecraft xyz velocity	Float(:,3)	Segment rate	6

[asteiker]

We determined that we will grab all variables within the /heights groups for each beam group. A new task was added to actually perform the subsetting using icepyx.

[asteiker]

This may still be helpful to determine core variables needed to create a derived product if we were to reformat to something like geoparquet.