record censored death values as missing rather than infinite

[corybrunson]

This PR revises the C++ source code to record death values beyond the threshold ("censored" deaths) as not-a-number rather than infinite and replace these with the missing double value Rcpp::NA_REAL (which corresponds to NA_real_) while populating the Rcpp::NumericMatrix.

The reason for taking two steps is that the std::vector storage of the persistent pairs cannot accommodate Rcpp::NA_REAL. One alternative would be to collect persistent pairs in a Rcpp::NumericMatrix throughout. Benchmark comparisons showed this option to incur significantly higher storage and runtime costs.

Coordinated with this ripserq PR.

[corybrunson]

@StrongKs @SeanHersh @resetdisconnect could you check that this branch installs and works on your machines? Feel free to experiment with the benchmark script but be cautious with the larger o3 data sets.