[ENH] downwards compatible exports of base_optimizer module (#718)

Adds back downwards compatible exports of `base_optimizer` module,
following #714

Author: fkiraly

pypfopt/base_optimizer.py

@@ -0,0 +1,9 @@
+"""Exports for _base_optimizer.py, for downwards compatibility."""
+
+from pypfopt.base._base_optimizer import (
+    BaseConvexOptimizer,
+    BaseOptimizer,
+    portfolio_performance,
+)
+
+__all__ = ["BaseOptimizer", "BaseConvexOptimizer", "portfolio_performance"]