Update interpolation input and output handling

[J-Hizzle]

Part of #47.

Changed the interpolation function to updateInterpolate, which now updates the data on a given target MultiHistogram with interpolated data from input MultiHistogram.

[J-Hizzle]

I just found that Cabana has some interpolation capabilities to begin with. Maybe we should fall back to those and get rid of my mediocre implementation eventually.

[Copilot]

Pull request overview

This PR refactors the interpolation utility to update an existing “target” data::MultiHistogram in-place using interpolated values from an “input” data::MultiHistogram, as part of the smooth thermodynamic force interpolation work in #47.

Changes:

• Replace the previous interpolate(...) -> MultiHistogram API with updateInterpolate(input, target) -> void.
• Update the interpolation implementation to write results into the provided target histogram grid.
• Update the unit test to validate the new in-place update behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
mrmd/util/interpolation.hpp	Updates the public API and documents the new in-place interpolation contract.
mrmd/util/interpolation.cpp	Implements in-place interpolation onto target’s grid using Kokkos parallelism.
mrmd/util/interpolation.test.cpp	Adapts the test to use updateInterpolate and compare against a reference histogram.

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