Reimplement purification-based density matrix extrapolation
Reimplement purification-based density matrix extrapolation

* The new code seems to work
* Reference:
  * The Journal of Chemical Physics 123, 044107 (2005)
  * The Journal of Chemical Physics 132, 124104 (2010)

william-dawson/NTPoly,william-dawson/NTPoly,william-dawson/NTPoly,william-dawson/NTPoly