add fit_seromodel_optimization function

[rccreswell]

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• 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)
• Checks have been run locally and pass

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

Preliminary work on allowing optimization in addition to MCMC sampling.

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

Users can infer parameters by MCMC sampling. (See #63, #73 ).

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

Users can find best fit parameters via rstan::optimizing.

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

Should not.

• Other information:

Replaces #170