Method to flush emission to CSV

[benoit-cty]

Addressing #217 after talking with @stas00 (see #235) I add a new method flush() that compute emissions and add them to the CSV, and/or call the API if you use it.
I use it in a Keras callback and it do the job of adding a line in the CSV after each epoch.
You can see it in mnist_callback.py

I've added documentation. But no unit test.

[stas00]

Thank you for adding this new API, @benoit-cty

I added a few language improvement suggestions.

[benoit-cty]

Thanks for the language improvement !
I've added unit tests so it's ready for review.

[stas00]

Tested this last incarnation to work correctly for our needs

Added a few tiny language corrections.

[vict0rsch]

Should we have run_id: Optional[str] = None in the EmissionsData data class? Right now the API test does not pass because of this

[vict0rsch]

Added on_csv_write: Optional[str]:

:param on_csv_write: "append" or "update". Whether to always append a new line
                        to the csv when writing or to update the existing `run_id`
                        row (useful when calling`tracker.flush()` manually).
                        Accepts one of "append" or "update".

[benoit-cty]

Nice additions Victor 👍

[stas00]

Thank you everybody who helped to make this happen!