NC-2499: Add BUILD_SHARED_LIBS and -TESTING-option

[martinweismann]

This PR intends to resolve #58 .

Only BUILD_SHARED_LIBS is standard in CMAKE (BUILD_STATIC_LIBS is no standard cmake-option). CMake default is OFF.

This pull request changes the cmake-project structure such that only one version (static or shared library) of lib3MF is built:

1. By default, the C-like interface is compiled into a shared library.
   (The reason to not include the C++-classes in the shared library is that this would decrease the portability of the shared library: bare C++ classes require matching runtime environments. It essentially puts all the restrictions a static library has into the shared library.)
2. If BUILD_SHARED_LIBS=OFF, lib3MF's C-like interface and the underlying C++ classes will be available in the static library.
   Users of the static library can choose which interface to use.

Also adds the option BUILD_TESTING to determine whether the tests of the library should be built. Default is ON.

[martinweismann]

@t-paul : this should conform to "standard cmake" now. Can you check in your project and comment on it? Thanks!