Add `event_binner` class for explicit binning. (#204)

* Add class for managing state associated with binning event times across integration periods.
* Include support for no or fixed 'regular' binning.
* Add a gtest-assertion compatible test for comparing sequences of floating point numbers: `testing::seq_almost_eq` in `tests/unit/common.hpp`.
* Rename `cell_` in `cell_group` to `lowered_`, to clarify intent (i.e. lowered cell state is very different from a `cell` object, and maintains state for many cells).
* Reformat some comments for consistency.

Note that the `event_binner` class is not used in this commit for actual binning: the original logic is still in place.