Set the biplot arrow scaling more appropriately

Previously scaled based on the metadata values. This meant that large metadata values would cause the arrow to be much larger than small metadata values.
Re-enable PD_whole_tree alpha metric

QIIME fixed their bug with slow PD_whole_tree calculation.
Allow wider variety of tags/barcodes, including *

'*' tag will use every sequence in a file, but only one sample per file can be defined.
Can now use numbers and other fun things for tags instead of 6/8 nucleotide barcodes.