Updated Rips zigzags
* Updated to use Bron-Kerbosch
* Compute in the order of increasing epsilon, and decreasing point sizes
* Split into plain zigzag (rips-zigzag) and image zigzag (rips-image-zigzag)
* Added minor enhancements (show_progress and timers)
Write an audit for the data structure.
-DBOOST_UBLAS_TYPE_CHECK=0 is necessary since otherwise Boost's uBLAS gives a compile-time error
when it doesn't find sqrt for polynomials. It doesn't actually need it (e.g., for LU-decomposition,
but it looks for it during type check). (File a bug report with Boost?)