v1.0.18

• Fixes bug where filled shares were incorrect for an order that 1) covered short shares and 2) would exceed the amount of cash available in the portfolio. The bug did not affect calculating any performance metrics, only the filled shares recorded for an order satisfying 1) and 2).
• Adds debug logging when a buy order amount exceeds the amount of cash available in the portfolio.
• Adds start and end date bounds checks to DataSource base class.
• Changes date type checks to use isinstance to support passing derived date types for start/end.