Optimized version of HaSa
1. We are now able to define the number of trees for randomForest.
2. Run in optimized mode 
  a. Improve the code to only use raster objects in memory. Clean the temporary storage.
  b. Use matrices instead of raster when possible.
3. Add a new sample method which only works for the optimized_mode since it runs with matrices.
4. Add `last_ref_val`, this is, the default reference value for the last step (default: 500)

Please check the documentation.