ENH: Provide a process_order method to override for custom slippage.
ENH: Provide a process_order method to override for custom slippage.

To make implementing a custom slippage model more straightforward,
provide a simulate method that will setup the calling of
`process_order`, which individual slippage models override to
do the unique slippage handling, where the simulate method handles
the boilerplate of checking order triggers, etc.

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