Exported functions for use in loo

[VisruthSK]

Summary

Exported some functions for use in the loo package. The two functions being exported are

• .pareto_smooth_tail()
• is_constant()

Closes #407.

Copyright and Licensing

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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/)

[jgabry]

Thanks for making the PR. Unit tests are running now.

[jgabry]

None of the failures appear to be related to this PR, so will go ahead and merge