Pydra

- fixing issues with quadratic memory increase for workflows with big splitters - removing `state_inputs` from `Task` (not used anymore)
- allowing for directory with broken symlinks for an input
- fixing tests so could be run on a cluster (using `tmpdir` as a `cache_dir`, so pytest can be run with `--basetemp`)
- fixing `cache_Dir` setting for worflows used as nodes
- fixing `Submitter._run_workflow` so it doesn't stop when there are still tasks in the graph, will try getting runnable tasks for another 60s (in case the previous tasks finished, but the results are still not available to read)
- fixing template formatting for files from `ShellCommandTask`, so it works for directories that have dots in the name
- fixing tests that were resubmitting tasks to the queue

- disable etelemetry in tests
- removing old pip (10.0.1) from testing (removing testallowfail from GA workflows)
- removing min_requirements
- adding rescheduling to the slurm worker for all jobs that are CANCELLED, TIMEOUT, or PREEMPTED
- adding generate_output_updates, to be able to predict names of the generated outputs before running

- allows README to be displayed on pypi
- limits testing to valid pytests

- fixing issue with `checksum_states` performance (it doesn't run `self.state.prepare_states` if not needed)

- moving from Travis to Github Actions for CI
- fixing issues with `numpy.arrays` provided as input fields
- adding a combiner option for dotfiles and graphs plots
- refactoring the input classes (adding `FunctionSpec`)
- adding `MultiInpObj` and `MultiOutObj` for a possible types in `input/output_spec`
- fixing `copyfile_workflow`
- setting proper `Workflow.output_spec` using tasks' `output_spec`s
- setting `output_dir` as a parent directory for all files created using the output templates
- adding `requires` key to `output_spec`'s metadata in order to set conditions for creating a specific output field
- adding `input_spec` and `output_spec` as class attributes that could be rewritten in the init or in the child classes

- refactoring template formatting for `input_spec`
- fixing issues with input fields with extension (and using them in templates)
- adding simple validators to input spec (using `attr.validator)
- adding `create_dotfile` for workflows, that creates graphs as dotfiles (can convert to other formats if dot available)
- adding a simple user guide with `input_spec` description
- expanding docstrings for `State`, `audit` and `messanger`
- updating syntax to newer python