illogical signatures and input tests in mscw/addtional-variables

[heikoklein]

Describe the bug Several functions in https://github.com/metno/pyaerocom/blob/main-dev/pyaerocom/io/mscw_ctm/additional_variables.py have illogical signatures and/or input-parameter tests, all marked with # pragma: no cover.

• https://github.com/metno/pyaerocom/blob/5aa165d29f55f7a3cb72435c1286b107e48392f6/pyaerocom/io/mscw_ctm/additional_variables.py#L56 Illogical varargs signature which then is enforced to exactly one arg, it should be change to

  def calc_concNhno3(conchno3: xr.DataArray) -> xr.DataArray:

  and then doesn't require further testing.

• https://github.com/metno/pyaerocom/blob/5aa165d29f55f7a3cb72435c1286b107e48392f6/pyaerocom/io/mscw_ctm/additional_variables.py#L130 same as above

• https://github.com/metno/pyaerocom/blob/5aa165d29f55f7a3cb72435c1286b107e48392f6/pyaerocom/io/mscw_ctm/additional_variables.py#L144 same as above

Some minor cases, which are stricter than really required:

• https://github.com/metno/pyaerocom/blob/5aa165d29f55f7a3cb72435c1286b107e48392f6/pyaerocom/io/mscw_ctm/additional_variables.py#L8 add_dataarray should be able to run with only 1 array as input. sum(3)=3 and very well defined. Currently, at least 2 arrays are required. Maybe even better without ValueError tests and a signature like

  def add_dataarrays(ar0: xrDataArray, *arrs: xr.DataArray) -> xr.DataArray:

• https://github.com/metno/pyaerocom/blob/5aa165d29f55f7a3cb72435c1286b107e48392f6/pyaerocom/io/mscw_ctm/additional_variables.py#L31 is also well defined with only one array. subtract(3)=3, subtract(3,1)= 2, ... Currently, at least 2 arrays are required. Could be solved as above.

[avaldebe]

xr.DataArray is a mutable object. Therefore, we should be careful to not to modify the input variable. Also, some of these functions would benefit from an assertion on the input units