Improve integration with fitdistrplus

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i.e maybe helper functions, better support for mixed censoring and mixed truncation (former more important than the latter) see the fitdistrplus vignette for current pain points.

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I think it makes sense to offer a wrapper (i.e fitdistdoublecens) to handle this vs a helper that then feeds into fitdist or fitdistcens. The reason I think this reason makes sense is that we need to pass quite a few custom things to fitdist that are fairly hard to pass and so it might be easier to do this for the user.