Added LTC1731 Li-Ion charger

Created by: ppielorz

Add LTC1731 battery charger:

• SO-8 4.1, 4.2, 8.2, 8.4 voltage versions,
• MSOP-8 4.1, 4.2, 8.2, 8.4 voltage versions.

Datasheets: 4.1, 4.2 voltage versions: https://www.analog.com/media/en/technical-documentation/data-sheets/1731f.pdf 8.2, 8.4 voltage versions: https://www.analog.com/media/en/technical-documentation/data-sheets/17318fs.pdf

Screenshots: SO-8:

MSOP-8:

CHRG pin (output) is put on the left according to datasheet and I think it is more reasonable.

---

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 symbol(s) you are contributing
• An example screenshot image is very helpful
• Ensure that the associated footprints match the official footprint library
  • A new fitting footprint must be submitted if the library does not yet contain one.
• Check the output of the Travis automated check scripts - fix any errors as required
• Give a reason behind any intentional library convention rule violation.