Add lfoN_beats

[jpcima]

Based on #364
This permits the phase of LFO to be driven by the musical clock.
It implements the opcode lfoN_beats.

Depending whether the opcode is used or not, and whether the hosts provides the musical clock, the code will select 1 of 2 paths for phase generation.
The value is taken to be the number of beats that span a period of the LFO.

Decoupling phase and waveform generation opens new possibilities for improvement.

• vectorize the wave generation
• implement lfoN_count, at the phase generator level