calculate_resistance calculates resistance between layers

[rubencalje]

Right now calculate_resistance calculates the resistance between layers (also see issue #134), and not the resistance of layers themselves (if that is correct english...).

I propose to rename the current calculate_resistance to calculate_resistance_between_layers, and create a new calculate_reistance with the formula:

c = nlmod.layers.calculate_thickness(ds) / ds['kv'].

Any objections?

[rubencalje]

Or add an optional argument to calculate_resistance named between_layers, which is set to False by default (which would change the current outcome of calculate_resistance). I prefer this option.

[OnnoEbbens]

Or add an optional argument to calculate_resistance named between_layers, which is set to False by default (which would change the current outcome of calculate_resistance). I prefer this option.

I also prefer this option