Issue #382: Reorganise function locations

[athowes]

Description

This PR will close #382. Creates:

• fit.R
• stancode.R
• validate.R

Removes:

• defaults.R
• generics.R

I think this is much clearer as a structure.

Follow up issues to create or add comments to existing issues:

• epidist_stancode.default can just pass back NULL or whatever it is does nothing. Doesn't need to abort.
• epidist_validate.default contains the catcher for data without a model class. Could make this error message more informative then?
• Do we want to export validate functions? Stan code functions? Users are not going to need to create these functions but they will need to create instances of the methods for their model

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.06%. Comparing base (d02c7c7) to head (3627b13).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
R/stancode.R	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #389      +/-   ##
==========================================
+ Coverage   88.55%   90.06%   +1.51%     
==========================================
  Files          11       15       +4     
  Lines         402      443      +41     
==========================================
+ Hits          356      399      +43     
+ Misses         46       44       -2     

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[seabbs]

Nice looks good

[seabbs]

Agree with the points for further issues.

Do we want to export validate functions? Stan code functions? Users are not going to need to create these functions but they will need to create instances of the methods for their model

IF we move to the validation model that scoring utils now uses we may be able to side step this discussion (validation .[ methods)

[seabbs]

Agree with the points for further issues.

Do we want to export validate functions? Stan code functions? Users are not going to need to create these functions but they will need to create instances of the methods for their model

IF we move to the validation model that scoring utils now uses we may be able to side step this discussion (validating.[ methods). As it stands I think we need to export it