fixes #534, simplifying fvar ctors

[bob-carpenter]

Submission Checklist

• Run unit tests: ./runTests.py test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary:

Simplified fvar<T> constructors to allow generic assignable value and tangent.

Intended Effect:

Remove all the complicated edge cases, avoid if_else, reducing the number of fvar constructors and simplifying them.

How to Verify:

Unit tests all still work.

Side Effects:

No.

Documentation:

Added doxygen doc to entire core/fvar.hpp file.

Reviewer Suggestions:

anyone

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):

Columbia University

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/)

[bob-carpenter]

Could someone review this one? Should be really easy as the new ones make sense and all the tests pass :-)

[syclik]

Thanks, Bob.