This commit enhances the type enforcement in the Shipping module by explicitly checking for method
existence before calls on variable types. In addition, it enforces argument type checks for several
function and method declarations to avoid type-related errors. Moreover, the error handling in
different functions has been improved by catching potential exceptions and logging them for debug
purposes. The commit also optimizes a PHP sorting function by leveraging the spaceship operator
instead of using conventional comparisons.

Related: #55

If a standard shipping is set, it is not automatically set to erp entities if these have linked erp entities.
In this case, these erp entities have been created from the others. the shipping has then been changed, deleted, etc.

* Implemented new ERP Entity Interface
* Extension of standard shipping costs: consider country of customer
  * Now when the address is changed AND the default shipping cost is assigned, the default shipping price will be adjusted accordingly

style: code style