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.

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. Now we have random_raster (equivalent to old random which uses raster::sampleRandom function), raster_regular (equivalent to old regular which uses raster::sampleRegular function), and random_matrix (new one which uses matrices and the stats::sample function over the only existent pixels and not over all as raster::sampleRandom - more info here).
  4. Add last_ref_val, this is, the default reference value for the last step (default: 500)