Rewrite introduction of paper

This commit rewrites the introduction as suggested by referee 2 and
by the editor.  Hopefully it is more clear.
Merge branch 'master' into ref2
Reword a few parts of the introduction

This commit tweaks the wording of a few parts of the introduction to make
minor improvements to its readability.
Merge branch 'ref2'
Make extremly minor tweak to wording in introduction
Merge branch 'master' into ref4