Option to reset/clear gin register

Currently, if a `@gin.configurable` is imported twice (e.g. through `importlib.reload()`), it raises an error.

This can be fixed by `gin.enter_interactive_mode()`, but interactive mode does not clear the previous scopes, which is potentially error prone as all `importlib.reload()` might interfere with one another. This makes it difficult to keep track of the current registered configurable state.

I would like to be able to clear to reset gin to it's initial state, so I am confident my colab will behave similarly to python script, without having to restart the colab runtime.