Migration to git

Finally all sources have been migrated from SVN to git. I decided to start with a fresh git repository without all the history from SVN. At the moment there are 3 relevant git branches: develop, qwt-6.2 and qwt-multiaxes: see https://sourceforge.net/p/qwt/git/ci/develop/tree/

The SVN repository remains being available if anyone needs code < Qwt 6.2: see https://sourceforge.net/p/qwt/code/HEAD/tree/