Move SDPOR core computation into a method

Move SDPOR core computation into a method

The core component of SDPOR involves selecting
an "initial" for a hypothetical computation
extending from a prefix of the current execution
that results in the reversal of the reversible
race SDPOR detected. This comomit moves that
computation as much as possible into the
`Execution` class itself to make the SDPOR
implementation easier to read