Enhance Rcpp::Nullable::as with explicit cast (Closes #1470)

[eddelbuettel]

As discussed in #1470 and building on this morning's discussion at the bottom of #1451, we can make Nullable<T> a little nicer. And that is all this PR does: for cases where the implicit as<>() needs a nudge, we can hide the nudge inside that (member function) as() function body accessing the matching as<> for the given T. A little bit of ellbow grease.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

Fine, although Nullable could be further improved. Having an as() method in the first place seems odd, because it deviates from the rest of the API. I.e., objects typically have T() and SEXP() operators. This one has the latter, and it would be nice to have the former for consistency instead of, or at least on top of, as(). Also typically set() and get() are private. See e.g. AttributeProxy, just to name something recent.

[eddelbuettel]

TBH I had forgotten these had as() and get() and I think they are barely used. The main use, and I just used it some more from another package, is just straight up instantiation as the T. I.e. in RQuantLib I let 'time to expiry' be either the double it always was (applied as 'time' relative to valuation date) or as a date and a number of functions now accomodate both types and then call this helper to determine the date:

QuantLib::Date getFutureDate(const QuantLib::Date today,
                             Rcpp::Nullable<double> maturity,
                             Rcpp::Nullable<QuantLib::Date> exDate) {
    if (exDate.isUsable()) {
        return Rcpp::as<QuantLib::Date>(exDate);
    } else if (maturity.isUsable()) {
        double mat = Rcpp::as<double>(maturity);
        // depending on the compile-time option, this is either intra-day or not
        // in minutes
        auto length = boost::posix_time::minutes(boost::uint64_t(mat * 360 * 24 * 60));
        return QuantLib::Date{today.dateTime() + length}; // high-res time ctos
    } else {
        Rcpp::stop("Excactly one of 'maturity' or 'exDate' needs to be supplied and be non-null.");
        return QuantLib::Date(); // not reached
    }
}

and I just realize I still need to rename today to evalDate or alike.