* Changes introduced with Rcpp/RcppTemplate Version 6.0 (last Version 5.3)

o Much cleaner object mapping mechanism between R and C++.

o New RcppZoo class to model Zoo time series objects (and via casting 
  ts, tseries, irts, and xts time series objects).

o New RcppFactor class to model R factors.

o New RcppDateTime class (a subclass of RcppDate) that models R's
  POSIXt date class (with time resolution down to fraction of second).
  Both date classes measure time in days (and possibly fractions of a day).
  A few date manipulation functions like nthWeekday(), isLeapYear(), etc.
  have been added (these can easily be extended to implement a financial
  date library). There are methods that can be used to return the value
  used by Excel (on a PC or a Mac) to represent the date value.

o The RcppFrame class has been rewritten to model an R data frame as a 
  vector of columns (the previous version used an awkward vector of rows
  representation).

o The RcppFunction class has been simplified and rewritten to support
  calling R functions with objects of any type that we can model in C++.

o An Rcpp namespace has been introduced to reduce namespace clutter.

o More careful checking of the types passed in from R has been added.

o References of the form x[i] = REAL(sexp)[i] make a function call for
  each i. This has been relpaced with double *rp = REAL(sexp), then
  loop over: x[i] = rp[i].
