Add posterior smoothing and posterior background to inform and estimate stages

[raphaelshirley]

** Feature request**

It is possible to improve the probability integral transform distribution shape and the negative log liklihood by fitting a Gaussian smoothing width and background flat probability to the posteriors. This could be fit in the inform stage and applied to the estimate stage.

This is a general feature that could be applied to all codes but is particularly useful in template fitting where it is not implicitly performed.

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• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
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