Enforce Qwt 6.1

Before we are not updating to Qwt 6.2 (see #207 (closed)), we should enforce Qwt 6.1 in our CMakeLists.txt. The Qwt project structure changes between these two version drastically, leading to compile errors (couldn't find <qwt_compat.h>). For users an error during CMake configuration might be easier to grasp and to solve.