Adding support for setting light attenuation in ADF file

[adnanmunawar]

Usage: In the light data, set the three optional normalized variables as

light1:
  namespace: /ambf/env/lights/
  name: light1
  location: {x: 0.0, y: 0.5, z: 2.5}
  direction: {x: 0, y: 0, z: -1.0}
  spot exponent: 0.3
  shadow quality: 0
  cutoff angle: 0.7
  attenuation: {constant: 1.0, linear: 0.0, quadratic: 0.0}  #<-- All these are optional i.e. any single one can be set by omitting others