Pydra

* adding sphinx documentation
* moving from `dataclasses` to `attrs`
* adding `container` flag to the `ShellCommandTask`
* fixing `cmdline`, `command_args` and `container_args` for tasks with states
* adding `CONTRIBUTING.md`
* fixing hash calculations for inputs with a list of files
* using `attr.NOTHING` for input that is not set

supporting tuple as a single element of an input

- fixing: nodes with states and input fields (from splitter) that are empty were failing

* big changes in `ShellTask`, `DockerTask` and `SingularityTask`
- customized input specification and output specification for `Task`s
- adding singularity checks to Travis CI
- binding all input files to the container

* changes in `Workflow`
- passing all outputs to the next node: `lzout.all_`
- fixing inner splitter

* allowing for `splitter` and `combiner` updates

* adding `etelementry` support

- Core dataflow creation and management API
- Distributed workers:
- concurrent futures
- SLURM
- Notebooks for Pydra concepts

Initial Pydra Dataflow Engine release.