Streamline coordinate system with emg3d

[prisae]

Currently, the coordinate system is quite flexible: RHS (positive z upwards) or LHS (positive z downwards), where depths can be increasing or decreasing:
https://empymod.emsig.xyz/en/stable/gallery/educational/coordinate_system.html

However, this has shown to be the source of a lot of confusion and many errors.

Streamline the coordinate system with emg3d, only one way how, RHS (positive z upwards):
https://emg3d.emsig.xyz/en/stable/manual/info.html#coordinate-system

What can be left flexible is if one defines the interfaces increasing or decreasing. E.g.

• [-1000, -500, 0] or
• [0, -500, -1000]
  The properties have to be provided in the same order.

Create a long deprecation cycle.

• 1 year DeprecationWarning
• 1 year FutureWarning
  or release v3.

[prisae]

Here the PR's where the "changes" were introduced:

• Left- and right-handed coordinate systems #70 (v1.10.4): LHS/RHS: NO CHANGE, only documentation (it was always possible).
• Make LHS/RHS direction independent #71 (v1.10.5): Only swap depth & properties, if provided in opposite direction

So this will probably not change: It was implemented originally a bit awkwardly (LHS), but it was never changed. Changing this is likely to create more problems than it solves, is backwards incompatible, and will not change anything in the end.

Better documentation could always help though...

[prisae]

Potentially related: #133