Move which depth/depth indices are read from code to config.

[BSchilperoort]

Currently the depths/depth indices that are read from the soil input data files are hardcoded in soil_io.py. These can be moved to a configuration file if desired so later.

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Let's move depths_indices = [0, 2, 4, 5, 6, 7] to prepare_soil_data and here consider it as an input argument to the function _read_soil_composition() as : _read_soil_composition(soil_data_path, lat, lon, depths_indices). We can add a ToDo to this module that the depths_indices can be read from config file.

Originally posted by @SarahAlidoost in #17 (comment)

Let's move depths = [0, 5, 30, 60, 100, 200] to prepare_soil_data and here consider it as an input argument to the function _read_hydraulic_parameters() as : _read_hydraulic_parameters(soil_data_path, lat, lon, depths). We can add a ToDo to this module that the depths can be read from config file.

Originally posted by @SarahAlidoost in #17 (comment)