[MRG] Implemented some Dataset loaders (#1)
[MRG] Implemented some Dataset loaders (#1)

First set of dataset loaders, for small sets of molecules.

```python
from cheml.datasets import load_...
```

should become the standard easy way of loading known datasets in a python environment. The loader should return a `sklearn.datasets.base.Bunch`, which inherits from `dict`, to expose the dataset in an intuitive way that is as coherent as possible across datasets.

Also: Inaugural commit!

CheML/CheML