Operator "Replacer" in island algorithms produces an exception

[abeham]

The Replacer is an operator that must be customized with two selection operators. In the default instance no selection operators are set and thus after a migration phase all population is lost and the algorithm stops with an exception. WorstReplacer and RandomReplacer are both a Replacer with the selection operators defined accordingly.

The idea of the replacement operators was to make them as generic as possible to allow for all kinds of (weird) replacement schemes. Basically, the replacement scheme in an Island GA works as follows:

1. In each populations select the emigrants using the EmigrantsSelector operator
2. Migrate the emigrants to another island through the Migrator operator (e.g. along an uni-directional ring)
3. Perform the immigration in each island
   1. Remove the "natives" that are to be replaced
   2. Select the emigrants that take their place
   3. Merge the remaining natives with the selected emigrants into one population

[abeham]

r5376

• Replacer will use Random replacement when no other replacement is defined

If the Replacer is used and it is detected that none or just one of the selection operators is defined, it will replace the missing ones with RandomSelection. It's not the best solution, but it completely satisfies the user's intention of not producing an error.

[swagner]

Adapted parameter descriptions in Replacer in r5408.