Default values of prior_weight

[sbfnk]

          The slighly broader issue is is that we would probably want a default of 0 for `estimate_infections` (weigh by number of data points) and 1 for `estimate_secondary` (don't weigh) but if we specify the prior weights with the distributions we don't know at the time of declaration which model this will be fed into and as what kind of distribution (gt/delay/etc.). Perhaps this should be a model option.

Originally posted by @sbfnk in #442 (comment)

[sbfnk]

Overall we probably want as default behaviour:

• in estimate_infections: uses generation time, truncation and delays
  • weigh all delay priors by number of data points (we probably want this for reproduction number / infection trajectory estimation but potentially not when focused on forecasting)
• in estimate_secondary: uses delays
  • don't weigh
• in estimate_truncation: uses truncation
  • don't weigh

Perhaps the simplest solution would be to make this a single boolean argument for each of the models, rather than specifying a weight individually by delay.