Support arbitrary data types

[matze]

Up to now, processing multi-dimensional single precision floating point data is the main target of the UFO framework, however support for element types other than that has been proposed. Here are some issues that we (@tomago, @rshkarin, @ashkarin, @kamer16) should discuss before implementing it:

• [x] What types are absolutely necessary? I suppose it's at least double.
• [ ] How should arbitrary kernels know about this? My first idea would be to compile the kernels with -Dbtype=double and use btype for all global arguments.
• [ ] What should we do if a certain type is not supported by the device? Especially double is a big problem on GPUs. We could convert implicitly but that's ... not what people would expect.
• [ ] Where do we set the desired buffer type? Per task? For the entire graph?