Edition 4 update 0

A new edition with some larger changes. See https://www.jirka.org/scv/changes.html for a complete list. The largest new bit is the addition of several sections in chapter 4 on solving the dbar problem in polydisc, extensions from affine subspaces, and the Cousin problems. A few of new theorems were added. In particular, the extension version Kontinuitatssatz was added in chapter 2, and we now actually prove that the singular set of a hypervariety is a subvariety in chapter 6. Also added are 66 new exercises. Downside is that exercise numbers have changed.