Create a cmake build scheme.

[mholtrop]

Create a CMakeLists.txt and accompanying files to allow Gemc to build smoothly using cmake.
Motivation is summarized nicely in Henry Schreiner's talk: CMake: Best practices, https://indico.jlab.org/event/420/#day-2021-03-02

To make this work really nicely, all dependencies (mlibrary, CCDB, evio...) should also be build with cmake and included as external dependencies.