MultiPop: Add prototype calcGenParamPop() and genParamPop()

[darizasu]

Description

This PR adds a prototype implementation of two new functions for population-level genetic parameter summaries on MultiPop objects:

• calcGenParamPop(): internal/helper function computing trait-specific summaries per population with a user-defined function.
• genParamPop(): user-facing wrapper that iterates across traits and returns population-level outputs (e.g. gv, bv, dd, variances/covariances currently implemented).

I also added basic roxygen documentation for both functions in R/popSummary.R (kept intentionally concise) and tests in tests/testthat/test-genParam.R (using Falconer's QG examples of genParam). These two functions were inspired by the functionality of calcGenParam and genParam.

The goal is to provide a first implementation for calculating genetic parameter summaries directly at the population level (including nested/partitioned MultiPop workflows).

Current behavior and scope

This implementation works as expected for linear summaries (e.g. FUN = mean, FUN = sum). It includes population-specific intercept handling so mean/sum behavior is consistent with the selected summary scale.

However, for non-linear summaries (e.g. FUN = prod, median, max), results should be interpreted with care, as the current decomposition is mathematically consistent for linear aggregation, but non-linear aggregators generally do not preserve the decomposition/equality with direct aggregation of individual total gv.

I'm making this a draft PR, so it's still a work-in-progress prototype.

@gregorgorjanc @gaynorr Please let me know your thoughts about it. I'd really appreciate feedback on the expected/desired behavior for non-linear FUN.

Related to #254

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Codecov Report

❌ Patch coverage is 77.12264% with 97 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (MultPop@dc9bb4f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
R/popSummary.R	44.84%	91 Missing ⚠️
R/selection.R	95.90%	5 Missing ⚠️
R/mergePops.R	99.25%	1 Missing ⚠️
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Additional details and impacted files

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