Add generic dnf inspired by UDFN2018-6 replaces #641

Scripted replacement for https://github.com/KiCad/kicad-footprints/pull/641 Script PR: https://github.com/pointhi/kicad-footprint-generator/pull/149

differences compared to manufacturer suggested footprint: larger toe fillet but slightly smaller side fillet. (heel the same as both have 0.2mm clearance to the EP)

parameters:

DFN-6-1EP_2x1.8mm_P0.5mm_EP1.2x1.6mm:
  device_type: 'DFN'
  size_source: 'https://www.diodes.com/assets/Package-Files/U-DFN2018-6.pdf'
  body_size_x:
    nominal: 2
    minimum: 1.95
    maximum: 2.075
  body_size_y:
    nominal: 1.8
    minimum: 1.750
    maximum: 1.875
  lead_width_min: 0.15
  lead_width_max: 0.25
  lead_len_min: 0.2
  lead_len_max: 0.3

  EP_size_x: 1.2
  EP_size_y: 1.6
  EP_num_paste_pads: [2, 2]
  heel_reduction: 0.1 #for relatively large EP pads (increase clearance)

  pitch: 0.5
  num_pins_x: 0
  num_pins_y: 3