Make plane coefficients public

[mhwasil]

We need model coefficient information to fit 3d bounding box given 2d bbox proposal. The bounding box calculation is defined here: bounding_box.
changes:

• Make model coefficients public
• Remove coefficient reference from findPlane function as it's public already.
  Other solution to avoid public variable
  
• Passing coefficient reference to segment_scene()

@minhnh @sthoduka

[mhwasil]

I pass a coeff reference to the function instead of making coefficients public.