Weight delays separately

[sbfnk]

Description

This PR closes #519.

It adds a weight_priors argument (having realise that "weigh" is the wrong verb!) to generation_time_opts(), delay_opts() and trunc_opts(). I'm not sure this is a super clear argument name but I haven't been able to come up with a better one.

The ..._opts() functions then add this to the delay distributions as an attribute. This necessitated some additional attribute copying in other parts of the code which is not great as it affects stability of the code but perhaps unavoidable.

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After the initial Pull Request

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[jamesmbaazam]

LGTM. I like that this option is now used at a more granular level, allowing for more control on the user side. I left a few suggestions in the documentation. Also, I think your choice of "weight" is correct and descriptive.