Move conversion functions to StructureAdapter header.

Also support calculations with StructureAdapter instances as well
as shared pointers.

This reverts commit c5cf10bf393dce44ce56b7d5b764121910d24fe9.
Use convertToStructureAdapter.

That one is the proper interface function.
(createStructureAdapter is a next level implementation)
Reorder methods as in the base class.

No change in code function.
Implement diff method for NoMetaStructureAdapter.

Calculate diff using the wrapped objects.