Make ADC16#snap be demux-aware
Make adc16_plot_chans.rb be demux-aware
Change plot options in adc16_plot_chans.rb

Now all plots with 8 subsplots are laid out as nx=2, ny=4.

The FFT plot now shows max bin number and total bin count.
Make ADC16Test demux aware
Add histogram convenience methods to ADC16Test
Set Vcom drive strength to maximum by default

Per Matt's recommendation, we now set register 0x50 to 0x30 (Vcom drive
strength to max), unless the user specifies it explicitly.
Add ADC16#est_clk_freq method
Add ADC16#rcs method (with ADC16#scm alias)
Add -g/--gain option to adc16_init.rb
Point all channels to input 2 for demux-by-4 mode

Unless the user explicitly writes to registers 0x3a and/or 0x3b, all
channels are set to point at input 2 when running in demux-by-4 mode
(aka single channel mode).