updates and new features

[bbalouki]

Bug Fixes:

• create_drawdowns now correctly handles empty pd.Series by returning defaults instead of raising an IndexError.
• markowitz_weights now falls back to an equal-weighted portfolio if all numerical solvers (SCS, ECOS, OSQP) fail to find an optimal solution.

Updates:

• markowitz_weights and the optimized_weights dispatcher now support a min_vol option for minimum volatility optimization.

New Features:

• Added the Black-Litterman model (black_litterman_weights) for more sophisticated portfolio optimization incorporating investor views.
• Added create_omega_ratio, create_calmar_ratio, and create_tail_ratio to the performance metrics suite.
• Added calculate_risk_metrics to provide Alpha, Beta, and Volatility relative to a benchmark.

Testing:

• New tests in tests/models/test_optimization.py and tests/btengine/test_performance.py cover all new logic and verified the bug fixes.
• Existing tests for events and data were verified to pass.