Write function for running optimization

[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. They could pass best fit parameters as the initialization of MCMC chains.

• 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

Draft PR, not ready for merging.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 15.78947% with 32 lines in your changes are missing coverage. Please review.

Project coverage is 69.15%. Comparing base (e4072c9) to head (194c99f).

❗ Current head 194c99f differs from pull request most recent head 9ca3192. Consider uploading reports for the commit 9ca3192 to get more accurate results

Files	Patch %	Lines
R/modelling.R	15.78%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #170      +/-   ##
==========================================
- Coverage   70.37%   69.15%   -1.23%     
==========================================
  Files          10       10              
  Lines        1779     1812      +33     
==========================================
+ Hits         1252     1253       +1     
- Misses        527      559      +32     

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[rccreswell]

Replaced by #189