- implemented global class registry (for class basic and derived classes)

- implemented archiving of expressions (except for class idx, and all classes
  derived from idx and indexed)
- added series() wrapper function
- class series renamed to pseries to avoid name clashes with global series()
  wrapper function, ex::series(), and basic::series()
- corrected the series expansion of single symbols with respect to the order
  term
- calling subs() on a series didn't work