Wave definition (offline rendering) improvements and changes

Value attributes now accept constant expressions.

Added 'duration' attribute (mutually exclusive to 'length') for
specifying the length of the wave in seconds rather than sample
frames.

Changed unit of 'samplerate' from kHz to Hz.

Attribute 'wavetype' now only accepts wave types - not values.

Updated rendertest.a2s and the megastrings.a2s example.