Add DLIB_CASSERT for optimization algorithms

[KamalElFeniche]

This PR adds an assert to check if the computed values are not NaNs. This is related to #1607.

[davisking]

Thanks, but this might break existing users code. E.g. someone could, by the current contract, have a function that is giving nans and be just ignoring it and taking the best result they found that wasn't nan. Which by the current contract they are allowed to do.

So better to let someone who doesn't want their objective to ever be nan to add that assert to their own output if that's what they want.

[davisking]

Er, wait, sorry, just read the other thread. It segfaulted? That segfault should just be fixed. Can you post an example that causes the segfault? Although that other thread is very old. Did you observe a segfault? What bad thing does this fix exactly?

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