Merge data_config and main config (or add overwrite mechanism)

[oceanscoper]

Currently data_config must be placed in ~/.grdwindinversion/data_config.yaml or os.path.dirname(grdwindinversion.file)/local_data_config.yaml

It is quite confusing, and not clear in the docs. It would be for more clear and easier to use to have only one config file, that can be passed as input.

I suggest merging data_config into the main config.