Replace R8 and R19 with 220k resistors

Because the 200k resistors are in parallel with a 2M resistor when connected,
the combined resistance is 1/(1/2e6 + 1/2e5)=182k. With the new 220k resistors,
the combined resistance is 198k which is closer to the intended 1/10 ratio
between gain steps. There should be no major change in function.
Merge commit '2250c1b' into master