added Texas_S-PVQFN-N16_EP2.1x2.1mm footprint

Created by: nivekg

Hey Guys,

I finally created the Texas_S-PVQFN-N16_EP2.1x2.1mm and Texas_S-PVQFN-N16_EP2.1x2.1mm_ThermalVias footprints for the INA3221 symbol (https://github.com/KiCad/kicad-symbols/pull/1343). Not sure if it conforms to the rules, I kept getting python errors when running check_kicad_mod.py

Datasheet: http://www.ti.com/lit/ds/symlink/ina3221.pdf#page=44 Screenshots:

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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.