Switch to an arrow-like partition structure for bundled data

[Bisaloo]

Data is partitioned across 3 axes and making this structure explicit by using an arrow-like partition structure may be helpful:

inst/extdata/
├── 2017
│   ├── all
│   ├── home
│   ├── other
│   ├── school
│   └── work
└── 2020
    ├── all
    │   ├── all
    │   ├── rural
    │   └── urban
    ├── home
    │   ├── all
    │   ├── rural
    │   └── urban
    ├── other
    │   ├── all
    │   ├── rural
    │   └── urban
    ├── school
    │   ├── all
    │   ├── rural
    │   └── urban
    └── work
        ├── all
        ├── rural
        └── urban

If we made this switch, we may as well store the data as parquet and use arrow for better performance.