Inconsistent pattern for passing tandem to constructor

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Related to #154

tandem is handled inconsistently in constructors. Sometimes it's an option, sometimes it's a required constructor parameter.

The pattern that I prefer is to use options, with default value tandem: Tandem.REQUIRED. Since just about everything eventually may need a tandem, using options prevents clutter in the constructor signature.

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Actually, the sim is quite consistent in how it passes tandem -- in all but one case (FocalPoint), it passes via a constructor parameter. This is not how I would have done it, but it's a legitimate alternative, and probably not worth changing at this point. So in the above commit, I just changed FocalPoint to match the pattern used elsewhere.

Closing.