Add left-truncation support in dpcens()

[zsusswein]

Is your feature request related to a problem? Please describe.
I frequently work with generation interval distributions and need to convert them from parametric specifications to probability mass functions. I can do some of this conversion with dpcens() but not all because dpcens() only supports right-truncation correction via D. I also need to left-truncate and renormalize my generation interval distribution PMF because the renewal equation assumes no same-day transmission so I need to truncate the lower bound of the PMF at 1 day.

Describe the solution you'd like
As suggested by @seabbs in this comment:

in some sense it could intoduce a API change (but I could handle with lifecycle) as maybe D should become D_lower and D_upperr or potentially even a 2 length vector (i.e (0, Inf) by default and then here (1, 27).

I like the 2-length vector idea. It feels clean to me.

Describe alternatives you've considered

The D_upper and D_lower argument idea would also add this functionality.

Additional context
Here's a reproducible script where I'm using dpcens() and manually applying the correction.

[seabbs]

Thanks for this @zsusswein. I agree this makes sense for both the R and stan code.

I think I agree that the D as a vector might be the best option. This throws up some thoughts:

• In R is length 1 allowed as this could be used to get the current behaviour?
• In Stan this will mean appending into a array or always passing a array (of length individual delays and with length 2. This could be a hassle but maybe its not that bad?