Improvement on memory usage

In the crossover function, the split point is chosen in the range of the
offspring size, instead of the solution size.

An out-of-range split point is silently ignored by the range selection:
it selects all genes from parent1, and none from parent2.
However, the error can be demonstrated by throwing an exception if the
split point is out of range.

Fix a typo in example_custom_operators

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