Clean-up of default value handling

[cehbrecht]

Description

While working on the OpenDAP integration (PR #394) we recognized that we have to change the current implementation for default values:

• We probably don't need to set default_type ... it is either fixed (Literal, BBox) or we guess it from the value. In case of ComplexInput it could potentially cause an error because we have several options (data, file, URL).
• Consistent handling of default for Literal, BBox and ComplexInput.

Steps to Reproduce

Additional Information