[External library porting] Make easier to force the semantic of an external class/struct

[lhamot]

For now, we can force an object to struct or class, but this is done in the main DPrint file.

It would be better to do it in an external cpp file, easier to extand.

The best is to put it in an external json file.

[lhamot]

A first step is done in 117011d
It is now easier to set the semantic in a (C++) CustomPrinter.
Soon in a yaml file.