Make Collection#each_pcm work with PMs yielding nil when no convertion exists (PM can't be converted to anything, PWM can't be converted to PCM and so on)
Make Collection#each_pcm work with PMs yielding nil when no convertion exists (PM can't be converted to anything, PWM can't be converted to PCM and so on)

VorontsovIE/bioinform,prijutme4ty/bioinform