Add weights for categorical exposures

[malcolmbarrett]

Closes #12

[Copilot]

Pull Request Overview

This PR adds comprehensive support for categorical exposures to propensity score weighting functions. The implementation enables weight calculation for multi-category treatments using matrix-based propensity scores and tilting functions specific to each estimand type.

Key changes include:

• Addition of categorical exposure support for all weight functions (ATE, ATT, ATU, ATM, ATO, Entropy)
• Implementation of propensity score matrix validation and column name handling
• Updates to error handling with new categorical-specific error classes

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/testthat/test-weights.R	Updates error classes from unsupported to matrix type errors for categorical exposures
tests/testthat/test-weights-categorical.R	Comprehensive test suite for categorical exposure weight calculations
tests/testthat/test-weights-categorical-comparison.R	Integration tests comparing results with WeightIt and PSweight packages
man/wt_ate.Rd	Documentation updates for categorical exposure support and focal parameter
R/weights.R	Core implementation of categorical weight calculations and estimand-specific logic
R/weights-utils.R	Utility functions for categorical exposure validation and matrix handling
DESCRIPTION	Addition of nnet package as suggested dependency