TRestMetadata::InstantiateChildMetadata method added

[jgalan]

Simply adding a method that will help to retrieve child TRestMetadata XML elements.

This will allow to keep TinyXml into the domain of TRestMetadata.

We can use that method to make the task of reading a child metadata element easier. I.e. I use inside TRestAxionXrayWindow as:

    if (fMask) {
        delete fMask;
        fMask = nullptr;
    }
    fMask = (TRestPatternMask*)this->GetChildMetadata("Mask", "mask");

[jgalan]

Needs review @nkx111 @lobis @juanangp