Note on calculating death rates for Strauss prior

[mmulvahill]

A note on the math for calculating Strauss death rates (calc_death_rates_strauss()). Deleting from comments and archiving reasoning here.

// Notes:
// Two possible version:
// 1) if we can use distr of birth_rate, it's simple -- just partial -
// full likelihoods.
// 2) if we have to use our set birth_rate (which I suspect based on
// Stephens2000), we have to multiply by papas^-1 * birth_rate
//
// Resolution:
// 1) is the correct approach. We aren't actually setting the birth rate.
// Instead we are providing the intensity parameter as the strauss and as
// long as death rate and birth rate intensity parms are the same, they
// cancel. Option 2) would require not using an accept/reject in birth step
// and instead generating from the actual Strauss density (i.e. really
// difficult and computationally expensive.
//