Decide policy of file names

[toshihikoyanase]

Motivation

During the review of #91, we found that we had no explicit file name policy.
In my understanding, we followed the following convention implicitly.

• Create {library_name}_simple.py that demonstrates the Optuna usage without the integration module (e.g., keras_simple.py).
• Create {library_name}_integration.py that demonstrates the Optuna usage with the integration module (e.g., keras_integration.py).

But some examples integrates the pruning example to simple example and switch the behavior with the --pruning option (See #77).

So, we may need to discuss the naming policy based on the current status.

Description

#77 will reveal the use of integration modules and existence of --pruning option in each example. So, this issue can be revisit after it.

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