3D-PDR Orion inspired dataset

This datasets contains a dataset of 8192 models of 3D-PDR in 1 dimensional mode. Each model was ran with a different configuration of initial physical parameters, namely number density, radiation field and cosmic ray ionisation. The respective ranges for these are: number densities $10^2 \leq n_\mathrm{H}(\mathrm{cm}^{-3}) \leq 10^7$ and lastly the cosmic ray ionisations $10^{-17} \leq \zeta(\mathrm{s}^{-1})\leq 10^{-15}$. Each of these parameter combinations is sampled from a Sobol sequence with the aformentioned minimum and maximum values.

Each model (grouped by `/model_1024` to `/model_9125`) consists of one parameter configuration related to its index in `model_df`, whose header is `radiation_field`, `density` and `cosmic ray ionisation`. 

Each model consists of a `cool`, `heat`, `line`, `opdp`, `pdr` and `spop` dataset. All datasets but the `pdr` are saved for completeness and relate to the cooling, heating and line populations of the models. More about those parameters can be found in the references on https://uclchem.github.io/3dpdr/, or upon request to the authors.

Most important are the `pdr` datasets, they contain the evolution of the physical conditions and chemistry as a function of the visual extinction going into an 1-dimensional cloud. 

The first 8 columns are respectively: `time_idx`, `positions`, `visual_extinction`, `tgas`, `tdust`, `etype`, `density` and `radfield`. Followed by all the species fractional abundances, these are stored under the `/species` key. 

Surrogate models for this code would need the following features: `visual_extinction`, `tgas`, `tdust`, `density`, `radfield`, and then any subset of species.