Various cleanup tasks

Multiple cleanup tasks. For most of these the 1-line commit comment explains what was done. The "DataBuffer c-contiguous" commit may need more explanation. Trying to emulate numpy.ndarray in C++ turned out to be a bad idea, because the task is too complicated in a language that doesn't natively implement slicing. DataBuffer itself probably worked. But any attempt at using non-contiguous buffers in the rest of OpenZGY would trigger several known and probably more unknown problems.