add scripted SO-20 Texas PowerPAD

[aewallin]

footprint guide: http://www.ti.com/lit/an/slma004b/slma004b.pdf
used by e.g. OPA569: http://www.ti.com/lit/ds/symlink/opa569.pdf (symbol PR is KiCad/kicad-symbols#1733)
script PR: pointhi/kicad-footprint-generator#340

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All contributions to the kicad library must follow the KiCad library convention

Thanks for creating a pull request to contribute to the KiCad libraries! To speed up integration of your PR, please check the following items:

• Provide a URL to a datasheet for the footprint(s) you are contributing
• An example screenshot image is very helpful
• If there are matching symbol or 3D model pull requests, provide link(s) as appropriate
• Check the output of the Travis automated check scripts - fix any errors as required
• Give a reason behind any intentional library convention rule violation.

[poeschlr]

See comments on the script pull request

[aewallin]

I have now revised this (pointhi/kicad-footprint-generator#340 New script pull request pointhi/kicad-footprint-generator#474) to follow the OPA569 datasheet Figure 16
http://www.ti.com/lit/ds/symlink/opa569.pdf

• mask opening is 3.56 mm x 4.47
• thermal vias 3 x 8 = 24 pcs, 13 mil diameter, with spacings from (http://www.ti.com/lit/an/slma004b/slma004b.pdf)

[poeschlr]

Looks good now, thanks