Add scripted TI S-PWSON-N8 (replaces #541)

Fixes #541.

This adds a footprint for TI S-PWSON-N8. The footprint uses normal qfn ipc settings (nearest result to suggested footprint). The suggested footprint is not reached as it seems to be quite old (TI suggests the long obsolete IPC-SM-782. telling me that datasheet is quite old or at least the suggested footprint is.)

The EP size is inclded as we did for similar footprints in the QFN library. (same custom format string as ti qfn footprints)

There is one more thermal via as the script does not support the generation of the exact pattern suggested by TI. (The drill size is a bit reduced so it should balance out.)

Datasheet link: http://www.ti.com/lit/ds/symlink/lp2951.pdf (drawing on page 27)

script PR: https://github.com/pointhi/kicad-footprint-generator/pull/244

size definition:

Texas_S-PWSON-N8:
  device_type: 'WSON'
  library: Package_SON
  size_source: 'http://www.ti.com/lit/ds/symlink/lp2951.pdf (page 27)'
  ipc_class: 'qfn' #'qfn' | 'qfn_pull_back'
  custom_name_format: 'Texas_S-PWSON-N{pincount}_EP{ep_size_x:g}x{ep_size_y:g}mm{vias:s}'
  body_size_x:
    nominal: 3
    tolerance: 0.1
   body_size_y:
    nominal: 3
    tolerance: 0.1
  lead_width:
    nominal: 0.25
    tolerance: 0.1
  lead_len:
    minimum: 0.4
    maximum: 0.6

  EP_size_x:
    nominal: 1.2
    tolerance: 0.1
  EP_size_y:
    nominal: 2
    tolerance: 0.1
  EP_num_paste_pads: [1, 2]

  thermal_vias:
    count: [2, 3]
    drill: 0.2
    paste_via_clearance: 0.1
    EP_paste_coverage: 0.75
    paste_avoid_via: False

  pitch: 0.5
  num_pins_x: 0
  num_pins_y: 4