Pydra

- fixing `hash_dir` function
- adding `get_available_cpus` to get the number of CPUs available to the current process or available on the system
- adding simple implementation for `BoshTask` that uses boutiques descriptor
- adding azure to CI
- fixing code for windows
- etelementry updates
- adding more verbose output for task `result` - returns values or indices for input fields
- adding an experimental implementation of Dask Worker (limited testing with ci)

- reorganization of the `State` class, fixing small issues with the class
- fixing some paths issues on windows os
- adding osx and window sto the travis runs (right now allowing for failures for windows)
- adding `PydraStateError` for exception in the `State` class
- small fixes to the hashing functions, adding more tests
- adding `hash_dir` to calculate hash for `Directory` type

- passing `wf.cache_locations` to the task
- using `rerun` from submitter to all task
- adding `test_rerun` and `propagate_rerun` for workflows
- fixing task with a full combiner
- adding `cont_dim` to specify dimensionality of the input variables (how much the input is nested)

* 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