Cached models only compared by name and tag

I have an issue with the current way we [load cached models](https://github.com/imagej/imagej-tensorflow/blob/master/src/main/java/net/imagej/tensorflow/DefaultTensorFlowService.java#L106). If one rebuilds the model locally or if we update remote networks but keep the name, the plugin will still use the old cached one because we just compare the file name and tag (right?). What would be the best way to do it? Are hash sums not appropriate for some reason? Could we at least check for a changed file timestamp? I can implement it, just asking for the best strategy or if I am misunderstanding something.