[ENH] yapf-ify code + add .gitignore

[kaczmarj]

This PR proposes:

• adding a .style.yapf file and updating the code according to yapf
• adding a .gitignore file

The .style.yapf file is an explicit listing of pep8 style. The changes that yapf made are reasonable, but please let me know if you don't like any of the style.

to run yapf, I ran this on the command line:

yapf -rip pydra

This will change files in-place and in parallel.

[kaczmarj]

This relates to #12 (comment)

Adding the .style.yapf file will prevent most (all?) user-specific differences in yapf style.

[codecov]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is 77.27%.

@@          Coverage Diff           @@
##           master     #15   +/-   ##
======================================
  Coverage    76.4%   76.4%           
======================================
  Files           6       6           
  Lines         856     856           
  Branches      228     228           
======================================
  Hits          654     654           
  Misses        141     141           
  Partials       61      61

Flag	Coverage Δ
#unittests	76.4% <77.27%> (ø)	⬆️

Impacted Files	Coverage Δ
pydra/__about__.py	100% <100%> (ø)	⬆️
pydra/engine/workers.py	92.72% <100%> (ø)	⬆️
pydra/engine/state.py	59.64% <37.5%> (ø)	⬆️
pydra/engine/submitter.py	88.8% <80%> (ø)	⬆️
pydra/engine/node.py	71.5% <81.57%> (ø)	⬆️
pydra/engine/auxiliary.py	74.86% <81.81%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a81e61...70e189b. Read the comment docs.

[effigies]

Can we exclude this from yapf? This is an auto-generated file.

[kaczmarj]

when merged, can these commits please be squashed?

[effigies]

Is this ready to add a flake8 check in Travis? I think we might need #14 first.