vol() method of the Transformer class should be able to handle negative gain values

Python package: sox==1.4.1
Python version: 3.6
OS: Windows 10
sox cmd line utility version: 14.4.2

The current implementation of the vol() method from the Transformer class throws an exception if the gain argument is set to a negative value. According to sox documentation from https://linux.die.net/man/1/sox it should be possible to set a negative gain value when is the gain type is selected as "amplitude" or "power".

One useful application of using a negative gain value is signal inversion. When I changed line 3558 as below and set the gain argument to -1 I was able to invert the input audio signal:

if gain_type in ['amplitude', 'power'] and gain < 0 and gain != -1:
raise ValueError(
"If gain_type = amplitude or power, gain must be positive."
)

Unless there is another way of performing signal inversion using another Transformer method, the above conditional statement can be modified to accept negative values.

Start at line 3558 in the Transformer.vol() implementation and compare its gain validation with the linked SoX documentation. Verify behavior for negative gain values with the amplitude and power gain types, including signal inversion with gain -1; done means valid negative values no longer raise the reported exception while other invalid values remain handled.