Mouse control multipliers fix

[adnanmunawar]

With this PR, users can set optional mouse control multipliers in the relevant camera description in the ADF files. These multipliers are camera-specific, and any specifier is optional. For example:

cameras: [camera1, camera2]

camera1:
  namespace: cameras/
  name: cam1
  location: {x: 4.0, y: 0.0, z: 2.0}
  look at: {x: 0.0, y: 0.0, z: -0.5}
  up: {x: 0.0, y: 0.0, z: 1.0}
  clipping plane: {near: 0.1, far: 10.0}
  field view angle: 0.8
  monitor: 0
  mouse control multipliers: {pan: 1.0, rotate: 1.0, scroll: 1.0, arcball: 1.0} # <--- All Optional multipliers set

camera2:
  namespace: cameras/
  name: cam2
  location: {x: 4.0, y: 0.0, z: 2.0}
  look at: {x: 0.0, y: 0.0, z: -0.5}
  up: {x: 0.0, y: 0.0, z: 1.0}
  clipping plane: {near: 0.1, far: 10.0}
  field view angle: 0.8
  monitor: 0
  mouse control multipliers: {pan: 1.0, arcball: 10.0} # <--- Some Optional multipliers set. Others assumed to be 1.0