gen_values: fixed FM sample repetition bug

In the previous version, the FM modulator issued the last sample of the
previous frequency-time tuple twice, once as the last sample of the
previous tuple, once as the first sample (sample = 0) of the next one.
added Robot 36 encoder
added SSTV.on_init() hook
optimized Robot36 to use PIL YUV converter
ColorSSTV: load raster data only once
GrayscaleSSTV: use PIL for grayscale conversion
grayscale: adjusted Robot24BW sync timing
bumped version to v0.2.3