Add age-varying normal model and change prior parameters specification

[ntorresd]

• Please check if the PR fulfills these requirements

• [x] I have read the CONTRIBUTING guidelines

• [x] A new item has been added to NEWS.md

• [ ] Tests for the changes have been added (for bug fixes / features)

• [ ] Docs have been added / updated (for bug fixes / features)

• [x] Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Add av_normal model without seroreversion and change initial prior parameters input specification in fit_seromodel.

• What is the current behavior? (You can also link to an open issue here)

Currently, the initial values of prior's parameters are specified in fit_seromodel as (e.g.):

fit_seromodel(
  serodata = serodata,
  foi_model = "tv_normal",
  foi_location = 0.1,
  foi_scale = 0.03,
)

• What is the new behavior (if this is a feature change)?

Now, it is specified as:

seromodel <- fit_seromodel(
  serodata = serodata,
  foi_model = "tv_normal",
  foi_parameters = list(
    foi_location = 0.1,
    foi_scale = 0.05
  )
)

Besides foi_model = "av_normal" is now a valid option corresponding to an age-varying model with normal for the force-of-infection. See changes in NEWS.md for further details.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

  • Add av_normal model without seroreversion.
  • Change initial prior parameters input specification in fit_seromodel

• Other information:

  • Note that the meaning of the parameters in foi_parameters may vary depending on the model.
  • Now it's possible to specify prior's parameters for the constant model.