Calculate real drawdowns

[chilango74]

It can be helpful in Portfolio and AssetList.

[chilango74]

Implemented in 0bea279 (dev branch).

New real_drawdowns property for both AssetList and Portfolio: drawdowns of the inflation-adjusted wealth index. Requires inflation=True (raises ValueError otherwise). It reveals purchasing-power losses hidden by nominal growth — e.g. an asset rising +0.1% per month with +0.5% monthly inflation has zero nominal drawdowns but steadily deepening real ones.

Covered by regression tests (test_real_drawdowns_reveal_losses_hidden_by_nominal_growth, test_real_drawdowns_match_inflation_adjusted_returns). Will ship with the next release.