SpatialGrid read_variable sorted output and associated issues

[alhom]

VlsvReader.read_variable, when called with cids=-1 (all CellIDs), returns data in the file layout, which can change between files. This keeps causing issues with users. Non-disruptive fixes to be done:

• investigate if we can warn when using read_variable from outside analysator vs. from within analysator -> warning to remind about sorting. __name__ doesn't really work though.

• Pass coordinates to read_variable -> use 0th-order read_interpolated_variable instead

  • this is piling extra tasks on read_variable, though.

• ✔️ read_interpolated_variable to have a 0th-order interp.

• read_variable_info to maybe sort by default? Returns VariableInfo. It is not a drop-in replacement for numpy ndarray, which is a bit unfortunate. Likely useful in any case, and non-disruptive (as long as sorting it doesn't break anything...)

  • this will need edits to most plotting scripts to account for not having to sort anymore. Will make for cleaner code, though!

What should be the user-facing function? Likely read_interpolated_variable (0th order), or read_variable with coords parameter.

[JonasSuni]

https://stackoverflow.com/questions/3711184/how-to-use-inspect-to-get-the-callers-info-from-callee-in-python

This discussion describes how to get the info of the function that is calling the current function. This way you should be able to implement a warning that triggers when the user is calling read_variable but not when other analysator functions are calling it

[alhom]

Piecewise constant read_intepolated_variable in here: #278