Atomica

- `at.ProgramSet` now stores all compartments with a `non_targetable` flag `in at.ProgramSet.comps` so that it can read/write workbooks for models that use coverage scenarios only.

- Exporting results now includes some time-aggregated quantities (summed over the year, rather than providing annualized values as of Jan 1)

- Added log level commands to `calibrate()` and `reconcile()` so that they respect `at.Quiet()` in the same way as `optimize()` already does.

- Update documentation to support Sphinx 3
- Added `__all__` variables to modules so that module variables are no longer accidentally imported into the top-level Atomica module.
- Renamed `defaults.py` module `demos.py`

*Backwards-compatibility notes*

- Code that relied on module variables being imported to Atomica may now fail. For example, `atomica.version` was accidentally set to the version string rather than referencing the `atomica.version` module. Such code should be updated by finding the relevant module variable and referencing that instead - for example, replacing `atomica.version` with `atomica.version.version`.
- Plotting settings are no longer imported to `atomica.settings` by accident - instead, they should be accessed via `atomica.plotting.settings` only. The same usage pattern applies to settings in other modules like calibration and cascades.

- `Project.calibrate()` no longer saves the new calibration to the project by default

*Backwards-compatibility notes*

- Add the explicit argument `save_to_project=True` to `calibrate()` to match previous behaviour

- Drop version constraint for `openpyxl` to support both version 2.5 and >2.5