Order of kinetic species in diagnostics

[EmilyBourne]

There is no reason for ions to be placed before electrons in the data_structure.yaml file:

gyselalibxx/post-process/PythonScripts/gysdata/data_structure_XVx.yaml

Lines 16 to 18 in adf6a14

	- &speciesd
	# get the coordinates of this dimension from a field in another file
	species: { constant_coord: [ "electrons", "ions" ] }

In fact depending on the main.cpp simulation file (or the .yaml describing the input) the order could be arbitrary. A solution could be for the code to output a list of string containing the names of each kinetic species ?

Besides, the data_structure.yaml is a rewriting of the pdi_out.yaml file in the simulation folder, and thus needs to be modified depending on the simulations parameters that we want to access during post process.

Issue transferred from GitLab, originally created by @yanmnc