Implemented ImageZigzagPersistence * Changed ZigzagPersistence to support a visitor, and implemented ImageZigzagPersistence * examples/rips/rips-zigzag now computes using ImageZigzagPersistence * PersistenceDiagram no longer records zero persistence pairs * Added utilities/memory.h with report_memory() function

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?)