demo build target

 My experimental code for FloatNeuralNet has grown kind of large, and it runs kind of slow with valgrind, which interferes with the normal tests. So I've added a new entity in the C++ code: "demo", and moved all the slower experimental examples there. They are just like tests, only sit in the "demo" subdirectories, in the files starting with "d_". The target to build them is "mkdemo", to run without valgrind "demo" or "qdemo", with valgrind "vdemo" (this is different from the tests where the target "test" runs with valgrind).

BTW, the target "mktest" didn't run the library build as a dependency. It does now, in the main cpp directory (but not in the subdirectories). And the same applies to "mkdemo".