Add 'level' to DatasetID

[djhoese]

Problem description

We currently have no way of describing data at certain pressure levels. In the NUCAPS reader I do something semi-hacky where I append the pressure level (in millibars) to the dataset name. This results in ~100 new dataset entries for each dataset that can only be identified by knowing the exact name. Due to how NUCAPS does things these datasets are 1D arrays iirc. It is also possible to request the generic dataset and get a 2D array that is (pressure level, rows * cols).

Proposed Solution

Add a level field to DatasetID. Default value can be None, otherwise it is a number of the millibars pressure level of the dataset. Biggest issue with this is we should really only support one type of "level": Do we do millibar pressure level or do we do altitude in meters?

[pnuu]

Often NWP data (and IASI L2 soundings) are on "hybrid" levels. Don't really know what they are, but there are 137 of them in the ECMWF model (same levels are used in IASI L2 data).

[djhoese]

What does "hybrid" mean in this case?

[pnuu]

Don't really know. That's what the NWP people call them.

Quick search forwarded me to this: https://en.wikipedia.org/wiki/Sigma_coordinate_system

[djhoese]

@pnuu If someone makes a vague request for a dataset (ex. by name, but it has multiple levels), should the highest level (highest pressure, lowest altitude) be returned or the lowest level (lowest pressure, highest altitude)?

[pnuu]

I would go for the lowest level (highest pressure, lowest altitude). But the user might except something else, e.g. when the data is something upper-atmosphere stuff. But at least the lowest level often shows something recognizable, like land/sea areas, clouds, etc.

[djhoese]

This was added by #283