Support DAG visualizations

See merge request !33

allow support for python 3.8 again

Increase test coverage

See merge request !32

Update output generated in README to reflect updated code/example

See merge request !31

Update changelog

See merge request !30

Have DAG.submit() return the submit result information

See merge request !29

Allow the creation of multiple duplicate layers

See merge request !28

this is potentially useful in the case of several dependent layers
with (nearly) identical submit descriptions and functionality, e.g.
merge/reduce patterns.

also drop support for python3.8 to allow type aliasing with primitive
types

Switch build backend, switch to CI components, add pre-commit config, add linter rules

See merge request !27

add API access for DAG submission

See merge request !26