Sample code for caching for faster DNAm loading

@sarudak I wasn't able to find a sample code to load data with caching parameters so I am adding this here:

```
cache_path='data/cache/'
cache=biolearn.cache.LocalFolderCache(cache_path,1000)
data_source = DataLibrary(cache=cache).get(geo)
```

Start by locating the documentation or examples for DNAm data loading, then review the DataLibrary and LocalFolderCache usage shown in the issue. Done means the caching sample is added in an appropriate place and clearly demonstrates loading data with cache parameters.

Written by the indexing model from the issue text.