Replace OPAMP with OPA1662 in MSOP-8 package

The previously selected opamps could only handle 6V, not the 10V we are feeding
them. Change the opamps to OPA1662, which have the desired characteristics.
Also, using dual opamps allows us to reduce the number of filter capacitors.

Because the change in footprint, re-laying part of the board was also needed.
The advantage is that the pinout of the MSOP-8 package is identical with many
other opamps.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reorganize ground planes

Use only one ground analog plane; do not split it up in mini ground planes for
power, reference voltage etc. The previous layout routed the ground, AIN, and
power signals independently, creating a bunch of small antennas ready to pick up
noise. To solve this, join the analog ground plane near the AIN pins on the
connector, and route all signals (including power) above the join.

Decouple the ground planes from the ground pin on the right-side connector.
Ground currents could have leaked around the analog ground plane, through the
GND pin, creating a large antenna. Not having a ground path through the right
GND pin would break compatibility with 20-pin boards. To solve this issue, add
an optional 0 Ohm resistor that connects the analog ground plane to the ground
plane on the right side.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>