Implicit NARs in Narwhal

Not debugged but seems straightforward: implicit sub narratives are implemented in Narwhal through the clunky api of

nar.makeImplicit()

The result is that all sub nars and all VARs at the bottom will have a self.explicit field set to False. The consequences is in GOF slot counting, where we use the "active" slots in the denominator instead of the total number of slots. The "used slots" is as before, the total "num slots" is as before. But now the "num active" is defined as found or explicit; and this is used in the denominator. This allows an unused and implicit part of the narrative to be ignored in the GOF - hence making the nar more multi-purpose.
There is a reward for filling implicit sub narratives: more words from the text are read.