Tutorial title change

[JaroCamphuijsen]

Currently the ESMValTool tutorial is called "ESMValTool Introduction". It says so at the top of the title page and at the top of each lesson. This also means that when users are starting section 01_introduction, the see at the top of the page:

ESMValTool Introduction
Introduction

I think this is not the best name for this tutorial since there are also advanced lessons at the end. Also this issue might be related to #22 since we might want these names to match.