Changed some resistor values in buffer amplifier following prototype
testing.  Added spreadsheet of measurements taken during testing.
Small changes to schematic.
Nil.
Changed buffer amp components to SMD.  Did some initial layout on buffer
amp.
More work on board layout and dimensions.
Added board clearances for case.  Added mounting hole for bracket to
rear panel.
Moved I2C pullup resistors from under Nano.
Tidied up bracket mounting hole.
Swapped pin allocations on two connectors, for consistency with rest of
board.
Updates to labelling.
Removed last of the dimension labels.
Tweaked some labels.
Final tweaks to traces.
Nil.
Nil.
Tweaking label.