Fixes the "Umbrella header for module Sift cannot be found"

[dbarden]

Purpose

Creates a include header that will contain the headers that should be distributed. It has some symlinks to the headers that should be public.

Summary

This avoid copying all the headers and solves the compile time error shown in #114

Testing

Integrate the sift package via SPM and compile the project

Checklist

• The change was thoroughly tested manually
• The change was covered with unit tests
• The change was tested with the integration example
• Necessary changes were made in the integration example (if applicable)
• New functionality is reflected in README (if applicable)

[viaskal-sift]

@dbarden the issue should have been addressed already, should we close this one?