TEST: remove useless files.
Merge branch 'master' of https://github.com/ahkab/ahkab into tests
Merge branch 'master' of https://github.com/ahkab/ahkab into tests
Update test pz1: add .OP
TEST: update test pz_spice: add OP, skip butterworth on Travis.
Remove double checking.
Add linearization of circuits in PZ analysis.
Merge branch 'master' of https://github.com/ahkab/ahkab into tests
Merge branch 'tests' of https://github.com/ahkab/ahkab into py3
PY3: replace filter, unicode.
PY3: future imports everywhere.