Added coercion generics

[gowerc]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Please describe the purpose of the pull request:

Closes #938 (Implements generic coercion functions for downstream packages to use)

Note that on local testing I got the following warning:

Warning (test-fit-mle.R:55:3): no error when checking estimates after failure
Fitting finished unexpectedly! Use the $output() method for more information.

Considering I didn't change any existing code (I just added generics for others to use, shouldn't have any impact on existing functionality at all) assuming this is either (a) expected or (b) not something I need to worry about?

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
F. Hoffmann-La Roche AG

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 86.57%. Comparing base (1b45e70) to head (f7b6918).

❗ Current head f7b6918 differs from pull request most recent head c8d6492. Consider uploading reports for the commit c8d6492 to get more accurate results

Files	Patch %	Lines
R/generics.R	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #943      +/-   ##
==========================================
- Coverage   88.33%   86.57%   -1.76%     
==========================================
  Files          12       13       +1     
  Lines        4553     4560       +7     
==========================================
- Hits         4022     3948      -74     
- Misses        531      612      +81     

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[gowerc]

Just to say that I don't think test failures have anything to do with the code change. Please let me know though if there are things you'd like me to adjust though

[jgabry]

Thanks! I agree that the failures are unrelated to this PR.

[jgabry]

I'm going to go ahead and merge this since the failures are unrelated. Thanks again @gowerc!