Merge pull request #664 from DanielVandH/patch-1

Clarify the meaning of `u_data` and `I` in the EnsembleProblem docs

Author: ChrisRackauckas

docs/src/features/ensemble.md

@@ -30,9 +30,13 @@ EnsembleProblem(prob::DEProblem;
     `rerun` was true this will be `2`, `3`, etc. counting the number of times
     problem `i` has been repeated.
   - `reduction`: This function determines how to reduce the data in each batch.
-    Defaults to appending the data from the batches. The second part of the output
-    determines whether the simulation has converged. If `true`, the simulation
-    will exit early. By default, this is always `false`.
+    Defaults to appending the `data` into `u`, initialised via `u_data`, from 
+    the batches. `I` is a range of indices giving the trajectories corresponding 
+    to the batches. The second part of the output determines whether the simulation 
+    has converged. If `true`, the simulation will exit early. By default, this is 
+    always `false`.
+  - `u_init`: The initial form of the object that gets updated in-place inside the 
+    `reduction` function.
   - `safetycopy`: Determines whether a safety `deepcopy` is called on the `prob`
     before the `prob_func`. By default, this is true for any user-given `prob_func`,
     as without this, modifying the arguments of something in the `prob_func`, such