Support leading comments in mmCIF #704

Lines starting with # are comments in cif files. Tools like PyMOL take advantage of this in their cif output, so we should definitely support it. However, BioJava (tested 4.2.8) currently requires data_ on the first line, and throws an exception:

ERROR org.biojava.nbio.structure.io.mmcif.SimpleMMcifParser - This does not look like a valid mmCIF file! The first line should start with 'data_', but is: '# generated by PyMOL 1.8.4.0'