Move local C++ code to an Arduino library.

Two steps:  Move the C++ code from the root directory into libraries/Elcano.  Adjust the includes to use the system path.

To use the new library:
Rebase from upstream.
Restart the Arduino IDE if it was open.
Sketch -> Import library -> Elcano.
That will cause the IDE to add all the include files.  Just delete those.
If you have files with any un-adjusted includes, see how the changes were done in the files changed in this pull request and do that.