Create MultipleRegressionBasics.md: First Example

material/3_wed/MultipleRegressionBasics.md

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+#Multiple Regression Basics
+
+## Motivation
+
+### Introductory Example: tree dataset from R
+
+[figure raw data]
+
+*Aim:* Find relationship between the *response variable* `volume`  and the *explanatory variable* `girth`?
+Can we predict the volume of a tree given its girth?
+
+[figure including a straight line]
+
+First Guess: There is a linear relation!
+
+
+## Simple Linear Regression