In abduce, allow switching to SIM_MANDATORY not at the sim root

[jefft0]

Currently, during simulated backward chaining, it is possible for a goal to have an "opposite" match to a RHS. In this case, the abduce method sets the simulation mode to SIM_MANDATORY:

case SIM_ROOT:
  sub_sim = new Sim(opposite ? SIM_MANDATORY : SIM_OPTIONAL, sim_thz, super_goal, opposite, ...);

Note that it does this when backward chaining first begins at the root. But it is possible to have an opposite match at later steps in the backward chaining, for example to match the anti-fact RHS of a strong requirement. We want this to create a mandatory solution also.

This pull request updates the logic in abduce to allow the simulation mode to switch to mandatory at steps other than the simulation root. If the simulation mode is SIM_OPTIONAL, it means that backward chaining has not previously had an opposite match, so we switch to SIM_MANDATORY if the current match is opposite, similar to the case for SIM_ROOT. If the simulation mode is already SIM_MANDATORY and the current match is not an opposite match, then abduce simply continues with the backward chaining.

However, if the simulation mode is already SIM_MANDATORY and the current match is opposite, then it means that backward chaining would have two opposite matches. We don't have a use case for this yet and it is not clear what the logic should be, so we don't allow this case.