Add EPSILON constant to CFLD example for improved maintainability

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This PR creates a new CFLD (Competitive Facility Location and Distribution) example and improves code maintainability by replacing a hardcoded magic number with a named constant.

Changes Made

1. Created examples/cfld.jl - A new example demonstrating the use of IPG.jl for competitive facility location problems
2. Added EPSILON = 1e-3 constant - Defined after other parameter definitions for better organization
3. Replaced hardcoded value in cfld_payoff function - Changed from self_costs[j] + others_costs[j] + 1e-3 to self_costs[j] + others_costs[j] + EPSILON

Benefits

• Improved maintainability: The regularization parameter is now defined in one place and can be easily modified
• Enhanced readability: The purpose of the constant is clear from its name rather than being a magic number
• Better documentation: The constant is defined alongside other parameters with clear context

Example Usage

The CFLD example demonstrates how to model competitive facility location games where players compete for customers while managing facility costs. The cfld_payoff function uses the EPSILON constant as a regularization term to avoid division by zero in cost calculations:

# Before: hardcoded magic number
regularized_cost = self_costs[j] + others_costs[j] + 1e-3

# After: named constant
regularized_cost = self_costs[j] + others_costs[j] + EPSILON

All existing tests continue to pass, ensuring no regressions were introduced.

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