Clean-ups and several bug fixes.

A big rework of the whole codebase.

Incorporate most changes from @weilawei.
Rename `ext_nXX` labels to `nXX` - this change is not backwards compatible. 
Enforce PEP8 everywhere.
Add transfer function results.
Fix setup.py
Fix DC sweeps.
Add .opinfo files. Correctly read back .op files (all previous verbose info is now sent to .opinfo)
Plotting bugfix for the latest matplotlib.
Sources have `ac_value` (complex) and `dc_value` (real).

Version bump, +0.011 version units.