* Add s-expression Format() function to all objects derived from
  BOARD_ITEM.
* Add s-expression Format() function to base objects as required.
* Add functions to convert coordinates from base internal units
  (nanometers) to millimeter string for writing to s-expression
  file.
* Add temporary dummy conversion functions to prevent link errors
  until schematic and board object and action code can be separated
  into DSO/DLL.
* Add CMake build option to build Pcbnew with nanometer internal
  units.