Document possibilities of positive z upwards and downwards

[prisae]

Change empymod from positive z downwards to positive z upwards, to be compatible with emg3d and most 'modern' geo-software and display tools.

Backwards incompatible, something for version 2.0.

See comment below, it is already possible. We just have to document it better:

• Manual
• Docstrings
• Example notebook

[prisae]

I gave it some more thoughts, and tested with some examples to confirm my guesses: It is already possible! As empymod does not make any assumption (not even of an air layer), it is completely agnostic to the direction of the z-axis, as long as depths are increasing. So

depth = [0, 300, 1000, 1100]
res = [2e14, 0.3, 1, 50, 1]

is a marine case, 300 m water layer, and a 50 Ohm.m target at 1 km depth. However, it could also be provided in the following way to empymod:

depth = [-1100, -1000, -300, 0]
res = [1, 50, 1, 0.3, 2e14]

The result is exactly the same. So empymod works for positive z downwards AND upwards, which is great news!

There are two differences to keep in mind:

• src/rec on a boundary: It is always taken to be in the layer towards the next lower interface, which is a different one depending on z upwards or downwards. To avoid confusion, put it a little bit into the layer. In the above example, if, e.g., rec_z=+/-300, it will be taken to be in the water layer if z is positive downwards, but in the subsurface if z is positive upwards. If you set rec_z=+/-299.99 it will be in the water layer in both cases.
• dip becomes -dip by switching from one to the other convention. azimuth remains the same. If positions are defined as [x1, x2, y1, y2, z1, z2] then it remains the same too.