First commit to the preprocessors section - just to check that everything is in order. This file is not finished yet

Author: rswamina

_episodes/preprocessor.md

@@ -0,0 +1,17 @@
+---
+title: "Working with preprocessors"
+teaching: 10
+exercises: 1
+questions:
+- “How do I set up a preprocessor?”
+- “Can I use different preprocessors for different variables?”
+- “Can I use different datasets for different variables?”
+Objectives:
+- “Create a recipe with multiple preprocessors”
+- “Use different preprocessors for different variables”
+- “Run a recipe with variables from different datasets”
+keypoints:
+- “A recipe can run different preprocessors at the same time.”
+- “The setting additional_datasets can be used to add a different dataset.”
+- “Variable groups are useful for defining different settings for different variables.”
+---