enable variable ratpm10pm25 via aeroval

[jgriesfeller]

This PR makes it possible to calculate the variable ratpm10pm25 via aeroval for observation.
The minimum snippet to use is this (example for AirNow obs network):

ODCSFUN = "AirNow;concpm10/AirNow;concpm25"
OBS_GROUNDBASED = {
    "AirNow": dict(
        obs_id="blablubb",
        obs_vars=["ratpm10pm25"],
        obs_type="ungridded",
        obs_vert_type="Surface",
        obs_merge_how={
            "ratpm10pm25": "eval",
        },
        obs_aux_requires={
            "ratpm10pm25": {
                "AirNow": ["concpm10", "concpm25"],
            }
        },
        obs_aux_funs={"ratpm10pm25": ODCSFUN},
        obs_aux_units={"ratpm10pm25": "1"},
    ),
}

Tests are still missing

[codecov]

Codecov Report

Merging #641 (541d99d) into main-dev (654ea63) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           main-dev     #641      +/-   ##
============================================
+ Coverage     76.89%   76.90%   +0.01%     
============================================
  Files            98       98              
  Lines         17644    17643       -1     
============================================
+ Hits          13568    13569       +1     
+ Misses         4076     4074       -2     

Flag	Coverage Δ
unittests	76.90% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
pyaerocom/ungriddeddata.py	61.79% <ø> (-0.03%)	⬇️
pyaerocom/units_helpers.py	100.00% <ø> (ø)
pyaerocom/_lowlevel_helpers.py	82.99% <0.00%> (+0.50%)	⬆️

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[jgriesfeller]

Needed because pyaerocom always tries to convert evrerything into the target unit first

[avaldebe]

I would appreciate some tests to cover the new unit helpers and the NotImplementedError obs filter

[avaldebe]

not sure if this lines are covered somewhere on the test suite, but it would be great to test that NotImplementedError can be used to filter obs

[jgriesfeller]

Not sure how to test these exceptions effectively. Any ideas?

[jgriesfeller]

The only ideas I have right now is a fake reader that throws these exceptions or a fabricated data set. Both takes some time to create.

[jgriesfeller]

I will extend these exceptions for the next release (e.g. with the exception raised when a station moved more than a kilometer).
How about postponing that test until then?

[jgriesfeller]

after speaking with @avaldebe : merging