Remove blah blah. Document based on control_sgtl500.h and
control_sgtl500.cpp.
Clarify that only one frequency modifier can be active at one.  Add
suggestions for pop-free ramping as indicated in datasheet.
Re-order function descriptions to group related ones and put in order of
signal flow.

Add #defines to control_sgtl5000.h to make selecting frequency modifier
(such as tone control) easier.

While changing everything else I also modified most things regarding
gain or volume from using 0 to 100 (or -100 thru 0 to 100) to using 1
(or -1 to 1) with only one exception I can think of right now in the
autoVolumeControl(..); - I modified all examples I found .volume or
similar in to suit.

I found opportunity to write these changes, I did not have a chance to
test much more extensively than just compiling it. I will have a chance
to test them by about 24 hours time.

The alternative route(..) was declared as unsigned short but was not
return anything.

Adding route() made me review dap_enable() more closely and it wasn't
doing what I intended to do before, it was ignoring I2S coming back from
Teensy, DAP was sourced from ADC as was I2S_DOUT and DAC was sourced
from DAP; Teensy received valid data from ADC but data from Teensy was
just discarded if user selected it.

unsigned short .adc_hpf(uint8_t bypass, uint8_t freeze);
unsigned short .adc_hpf(uint8_t bypass);
bypass & freeze as defined on page 34 of the SGTL5000 datasheet.

added last of dap 'bits' to control_sgtl5000.* and include an example
for bass enhance, need to make one for surround stuff.

Added
AudioControlSGTL5000::dap_avc(..);
AudioControlSGTL5000::dap_avc_enable(..);

SGTL5000_Specific/dap_avc_agc example sketch

Also added 'public domain...' line to all my examples.