Generalize GeoLifeCLEF raster loader

[tlarcher]

• Context
  The GeoLifeCLEF challenge was a main subject to adress with the Malpolon framework and custom raster & patch loaders were implemented in malpolon > data > datasets > geolifeclef.py

• Problem
  These specific loaders cannot be reused or called as they are to load other data. On the other hand, implementing new custom raster and patch loaders for each new dataset would most probably result in code redundancy.

• Solution
  A generic raster and patch loader should be written, allowing easy import and inheritance of common data structures and parameters.

[tlarcher]

In addition to the recent updates:

• easily merge dataloaders of rasters and jpeg in 1 model
• provide such an example for it