Clean old parts from threedigrid

[caspervdw]

Now that we have a new route to produce gridadmin files, there are large parts in threedigrid that are not used anymore. Cleaning these will greatly reduce complexity of this library, making it more maintainable on the long term.

This ticket is to investigate which parts are obsolete. I think it is good to aim for a breaking release that only supports the new route. It is also good practice to explicitly list the public api (eg in api.py) so that we know where we need backwards compatibility.

A start of obsolete stuff:

• “prepare” functionality
• unused fields
• lines.breaches
• Id mapping (seq_id fields)