MultiStatistics class supports different functions for each Statistics object

[TianYuanSX]

Thanks for making such a good EC platform!

Currently, class MultiStatistics can contain multi Statistics object, but all Statistics object must registered the same function.
Sometimes, we may need to assign different functions for each Statistic object.
For example, a Genetic Programming algorithm, if I want to analysis the terminal occurrence of the current population, I need to count the occurrences of each terminal, the statistics result looks like a dict {"terminalA": 10, "terminalB":18, ...} .
Besides, I also want to record the min/max/average fitness.
If I use a MultiStatistics object, the numpy.mean/min/max/std works for fitness, but doesn't works for counting the terminal occurrences (a customized sum is needed).

Of course, I could create seperate Statistics objects that can assign respective funtions but I need to change the signature/arguments of algorithms to allow multi Statistics objects.

Could the Statistics objects in MultiStatistics object register different functions? Or is there any good solutions for this issue?