Use default log in Inversion class (#12)

Use a default log in Inversion if `log=True`, so the user doesn't need
to build it and pass it. If `log=False`, then `Inversion.log` will be
`None`. Users can still pass their custom `InversionLog`. Move the first
call of `log.update()` to the `__next__` method, only executed in the
first iteration. This makes Inversion instantiation faster (no calls to
objective functions), and allows users to edit the log columns before
the inversion has started. Rerun notebooks.

Author: santisoler