Possibility to add observables in the first external process

[jgalan]

We (me and @Anakintana) are trying to add an observable at the process TRestRawBiPoToSignalProcess but it seems that it is not properly adding the observable.

I added some debugging output inside TRestEventProcess::ReadObservables.

But it seems that the analysis tree does not contain the entry even though it seems the <observable entry at the RML is properly processed.

It is possible that in the present framework implementation we are not allowed to add observables in an external process? Or I got anything wrong? If it is not possible at the present implementation which would be a good way to bypass it?

Probably @nkx111 has some insights?