Codontable bugs.

[Juke34]

Fix bugs in codon table:

• table 0 was not selectable via Bio::Tools::CodonTable->new( -id => 0); but was working only by creating a codon table by default and using the id function to set the table to 0
• table 7-8 and 17-20 was selectable while have been removed
• Negative value was usable value while it should not