dynamic_sizing fields

Hi,

First, thanks for this repo.
I may be wrong, but for datasets `flag_dynamic_sizing` and `sphere_dynamic_sizing` (download through the `.sh` script), the `dynamic_sizing` field in the `.json` file has strange attributes:

```
"sizing_field": {
"type": "dynamic_varlen",
"shape": [
-1,
4
],
"dtype": ""
}
```

This raises a first attribute error due to the malformed `dtype`, then a second reshape error.
Modifying to:

```
"sizing_field": {
"type": "dynamic_varlen",
"shape": [
-1,
3
],
"dtype": "float32"
}
```

seems to solve the problem.