Default behaviour of `copy_field`/`move_field`/`add_field` should be destructive, the appending behaviour creates problems

[TobiasNx]

See here
(The error seems to be not virulent any more, but the underlying behaviour is still productive.)

Catmandu Fix default behaviour is destructive

Appending should only be done explicitly and copy/move/add_field should be destructive.
Could be that this is the root of #113

• add_field (Change add_field() Fix function to destructive behaviour for Catmandu compatibility. #308)
• copy_field (Change copy_field()/move_field() Fix functions to destructive behaviour for Catmandu compatibility. #313)
• move_field (Change copy_field()/move_field() Fix functions to destructive behaviour for Catmandu compatibility. #313)

[blackwinter]

Catmandu is destructive: hbz/Catmandu@4d2f0e3.

[fsteeg]

To avoid conflict with #102, should be started after the planned first PR for #102.

Functional review: @TobiasNx
Code review: @fsteeg