[Feature] Asset allocation tools

[redstreet]

Nice to have additions to asset allocation by class module:

• compute how to allocate a given fixed amount to move one's AA close to their target (contribution)
• compute, for a given amount, what assets to sell to maintain or move one's AA close to their target (drawdown)
• compute recommendations to sell/purchase assets to rebalance so one can move to one's target AA (while minimizing capital gains)