Naming convention and I/O interface

In this merge request, I would like to unify the naming conversion which is being used in pleque and improve the input/output interface.

todo:

• finish naming_conventions.rst document and implement it to the documentation page
• implement new convention to all places in the code (note: this should be done at one moment after the previous task, maybe in separate merge request)
• prepare xarray <-> dictionary functions
  • -> direction
  • <- direction
• chart a workflow of format conversions
• implement toxarray function for Equilibrium, FluxSurf, FluxFunc and Coordinates
• implement OMAS2xarray function
• prepare example of ITER IMAS -> OMAS -> xarray -> Pleque

(!) Last two points are of crutial importance, since I would like very soon present pleque on ITER.