Best weight check between training sessions and exported set_batch_network

[keko950]

Just modified detector.c in order to check the best weight map every time a training session is started as described in https://github.com/AlexeyAB/darknet/issues/3452.

If *best.weights exist in backup path, detector will do a map before start training, it will save it and initialize the best_map value, so now you can maintain your best weight across different training sessions :)

Edit: Also updated network.h in order to add set_batch_network to the exported api, so you can use dynamic batching..