Enable handling of TEI-documents located in a sub-directory

We need to cover this file-structure:

transcript/
├── collection/
│   ├── 4354/
│   │   ├── ts_std_mono_04354.xml
│   │   ├── images/
│   ├── 4355/
│   │   ├── ts_std_mono_04355.xml
│   │   ├── images/
│   ├── 4356/
│   │   ├── ts_std_mono_04356.xml
│   │   ├── images/
│   ├── 4357/
│   │   ├── ts_std_mono_04357.xml
│   │   ├── images/

So, we can 'give' tg_model the collection-path and define, that it should collect TEI-files in the sub-directory