Enforce 'FCModel tables must have a single-column primary key'

Say I create a 3-column key,

Before the commit
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`isPK` returns 1 on three passes, `primaryKeyColumnCount ` is set to `1`, contrary to its name. No exception.

After the commit
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 `isPK` returns 1 on three passes, `primaryKeyColumnCount ` is incremented 3 times. At the end of the cycle, an exception informs me that multi-column keys are not supported.