CircuitBase class is finished, as well as functions to connect and stack circuits.  Merging circuits unfinished. Doctests only.
merge_circuits works. Need unit tests for circuits.base, but otherwise complete.
Refactor CircuitBase to Circuit
Output bits can be wired to multiple inputs, so discarded all use of occupied_outputs in components.
Merge with multiple output-wiring hotfix
Unit test for Circuit class
Merge branch 'master' into circuits
Merge with circuits