[T21] Specify standard reference system on subsurface

[santisoler]

In order to make any to_subsurface() or from_subsurface() function to work seamlessly on every library that wants to implement them, we would need to determine a standard reference system on subsurface. This way, if a given library A converts their data structures to a subsurface object, another library B can open it without having to worry about how A defines their coordinates.

After a brainstorming with @Leguark , we ended up in some conclusions:

• subsurface should use a right-handed coordinate system with z axis point upwards and x and y axis as horizontal ones.
• There will be no assumption on the direction of x and y coordinates: they can either be pointing towards east and north, respectively, or towards southwest and southeast, respectively.
• Each library should manage their own coordinate conversions on their to_subsurface() and from_subsurface() functions.

These choices are based on the default reference system in PyVista, which sounds very rational. This means that we would keep a fairly easy way to plot subsurface objects in PyVista.

What do you think?