AST-699 Use Skymodel in the demixer step

This fixes an issue where an empty pattern in the sources_pattern behaves differently with the new sourcedb class. The original code did a comparison of the name, the new code uses a helper function. This function handles an empty string as a wildcard.

The Demixer step now accepts a skymodel text file and the tests have been adjusted to use this text file. In order to get the skymodel from the original sourcedb some modification were made to showsourcedb.

Note all patches in this branch have a specific purpose, therefore the branch won't be squashed.