Non existing properties read write

As discovered by @blanchonvincent in #122, writing and reading non-existing public properties leads to weird errors.

This PR aims at fixing that.

 - [x] Demonstrate failure
 - [x] Introduce failing tests for all the proxies affected by the error
 - [x] Introduce fix to allow writing on non-existing properties

The bug likely affects only ghost objects and the scope localizer, but tests should verify that.
Performance should not be a big issue here.