#1 Add VPM algorithm

[Mark Baijens]

The VPM algorithm has been undergone two major updates since the programs available on this web site were written. The first updates, made in 1999-2001 extend the VPM to repetitive diving, as detailed in the publications by Yount, Maiken, and Baker (this has been dubbed VPM-A). In 2003, Baker introduced VPM-B, which systematically decreases ascent gradients, thereby extending shallow stop times.