Add nasced.c

[oleole39]

Hello @twilly ,

This PR fixes #43 by integrating nasced.c source uploaded by @Robert-github-com and tested successfully by me using a file in HQ quality downloaded in raw format from a VN3100PC device:

• Makefile in sandec folder is configured to used this file by default.
• Makefile in root folder is configured to build sandec together with odvr to make things easier.
• A bash script was added to help converting all raw files at once after they are downloaded using odvr inside odvr folder.
• README in main folder was updated to reflect those changes
• A README was added to sandec folder to reflect those changes and explain what are the differences between sandec.c and nasced.c

This PR does not address the facts that in my case (Ubuntu 22.04 based):

• odvr-gui is not functional
• 41-odvr.rules are not working (i tried changing the group to one my user was in and permission to 0666 then reload rules but without success).

Still CLI implementation of odvr works flawlessly when run with sudo odvr.

Thanks again for this piece of software which I have been very happy to find a few years after...