Use tiled exogenous data

Accessing "exogenous data" in custom features was added in #197 (closed). The documentation says An additional to be used for external features which I interpret as "the path to a tif image containing the exogenous data". However, we would like to use tiled data for example to provide the distance to a cloud for each pixel and each date.

Maybe the userFeat would be more relevant for this use (?), but it's not compatible with custom feature yet.