Additional DAC Controls

[relic-se]

Add the following additional DAC-related controls:

enableDacSoftMute
disableDacSoftMute
enableDacSlowSoftMute
disableDacSlowSoftMute
setDacDeEmphasis
enableDacSlopingStopbandFilter
disableDacSlopingStopbandFilter

Notes:

• Though the de-emphasis setting is intended for CD audio output, I've found it helpful to clean up the output of Adc signals that have been processed by an MCU and then sent back through the Dac (ie: effects).
• The soft mute settings successfully remove popping noises when enableDacMute is called.
• Though, the naming conventions I chose are wordy. Advice on better names for these methods would be appreciated.
• I haven't noticed a significant effect on the output of the Dac with the sloping stopband filter enabled. The science behind these filters is foreign to me, so I'm not positive exactly what benefits there are to gain with this setting. I'd be happy to remove those functions from this PR if it would be preferred to do so in order to avoid confusion.