Proxy fields usually reads and transforms data from other sources, usually other Pdm fields. This commit disables IO for proxy fields in the constructor. If XML serialization is required, this must be set on the proxy field, like this
field.xmlCapability()->setIOWritable(true)
field.xmlCapability()->setIOReadable(true)
* Refactor interface to PdmObjectHandle and PdmFieldHandle
Return objects instead of passing in structures as parameters
* Add nodiscard to several functions
* Remove redundant this->
* Rename to ptrReferencedObjectsByType