Add warning for when micrograph image names include the file extension

[tbepler]

Including the file extension in the micrograph name causes topaz to fail to find micrographs. See for example https://github.com/tbepler/topaz/blob/master/topaz/commands/train_test_split.py#L30

It would be great if topaz could correct these names automatically, but a warning when micrograph names contain "." is a good place to start.