[wip] Update cmake configuraton to targets based approach

[tmadlener]

BEGINRELEASENOTES

• Upgrade the cmake configuration of LCIO to the newer target based approach. That should make it easier to use LCIO as a dependency for other packages. Additionally it should become easier to remove quite a bit of cmake macro code that took charge of the dependency management, since that can now be handled by plain cmake.
• Bump the minimum cmake version to 3.14

ENDRELEASENOTES

Fixes #133

• Check that the "legacy variables" that are currently exported are really enough for downstream packages to use LCIO
  • Successfully built Marlin on top of this.
• Check which cmake macros have become obsolete and remove them
  • MacroCheckPackageVersion.h is still used by FindCLHEP.cmake so that would need to be touched as well. Keeping it for the moment.
• Check if all the build options that are available still work
• TODOs from CMake configuration overhaul #133
  • Use FindPython
  • Make BUILD_TESTING behave as expected
• root dict generation. At least avoid re-generation of these every build even when nothing changed.