Jobflow

- Enable serialisation of bson.

- Move project configuration to `pyproject.toml`.
- Add tutorial on generalized makers (jmmshn, 268)

New features

- Delayed updates to config and metadata for dynamic flows. See docstring of
`Job.update_metadata` for more details (gpetretto, 198)
- Additional stores are now generated on the fly as memory stores if they are not
specified in jobflow settings (davidwaroquiers, 183)

Bug fixes:

- Optimised calls to update_kwargs (jmmshn, 177)
- "job_uuid" and "job_index" are now indexed fields in additional stores (jmmshn, 165)
- Fix additional store storing `None` (mjwen, 160)

Enhancements:

- Docs refactored (utf).
- Added code of conduct (utf).

New features:

- New ``update_metadata`` function for updating the metadata of jobs and flows.
- New ``update_config`` function for updating the config (included manager_config) of
jobs and flows.
- New ``DIRECTORY_FORMAT`` option in JobflowSettings for controlling the date time format
used to create new directories.
- New functions for adding and removing jobs from a flow. The ``Job.jobs`` list is no
longer mutable (gpetretto).
- New ``Job.hosts`` attribute that stores a list of all host Flows. This captures the
nested nature of flows with the outer flow always first in the list (gpetretto).

Bug fixes:

- OutputReferences are no longer iterable.
- Docstring clarifications (utf, mjwen).

New features:

- Validate subschemas of nested models (gpetretto, 118).
- ``downstream_manager_config`` for controlling config of dynamic jobs (arosen93, 121).
- S3Store yaml parsing (jmmshn, 124).

Bug fixes:

- Fix home path for loading settings (gpetretto, 119).
- Docs updates (arosen93, 111).

Bug fixes:

- Docs fixes (arosen93).
- Compatability with maggma>=0.38.1 (68)
- Fixed missing PyYAML requirement (67)