audiotestsrc: Improvements to the "ticks" wave

(this is actually the work of Carlos Rafael Giani, coming from #305)

The ticks waveform can be useful for audio synchronization diagnostics and other cases where the time offset between waveforms is important. However, in its current form, it is too limited, and has problems with discontinuities, which result in severe artifacts when this waveform is output by a DAC.

This patch fixes some discontinuities and considerably expand the ticks waveform's flexibility. They also introduce the notion of a "marker tick"; every Nth tick can have a different amplitude (usually one that is larger than the others). This is useful for combining frequent oscilloscope triggering with large time offset detection. For example, without marker ticks, the tick intervals must not be too small, otherwise the maximum time offset that can be unambiguously detected is quite small (for example, if the interval is 50ms, then no time offset larger than 25ms can be unambiguously recognized). If the tick intervals are too far apart, then no sudden changes can be clearly observed, since the oscilloscope is not updated quickly enough. But with marker ticks, this is not an issue: If there's for example a tick every 100 ms, then the oscilloscope can be triggered every 100 ms. And, if every 20th tick is a marker tick, then time offsets of up to 1 second can be discovered, even though the time between ticks is 100 ms.

The patch also applies some minor cleanup to the audiotestsrc documentation.