audioadapter should have ability to fill in gaps with values other than zero

Other options should be:

• "const": arbitrary constant
• "nan": NaN (for floats)
• "hold": last valid value

I assume this is just a matter of providing an alternate "zeros_func" function to the buffer.filleddata() call:

https://git.ligo.org/greg/sgn-ts/-/blob/main/src/sgnts/base/audioadapter.py?ref_type=heads#L94

The argument to buffer.filleddata() should also be changed to something generic like "fill_func":

https://git.ligo.org/greg/sgn-ts/-/blob/main/src/sgnts/base/buffer.py?ref_type=heads#L413