Updates to structure project, conversion to slf4j and minor bug fixes

Conversion of org.biojava.bio.structure package (main) in biojava3-structure project to slf4j.  Minor bug fixes including printing array contents instead of address, handling potential null pointer exception
One important point here and in general for all demos: we should really keep System.out.print for demos as they intend to be simple and runnable even if you have no logging setup (which is perfectly possible if you just get the biojava jars and try one of the demos)