Gdsfactory

- Works now for python>=3.6, before only worked for python3.7 due to [type annotations](https://www.python.org/dev/peps/pep-0563/)
- nicer netlist representations (adding location to each node in the graph)
- YAML loader accepts strings (no more io.StringIO)
- better docs
- add_tapers only tapers optical ports in the new containered component
- add_ports from polygon markers
- add_io_optical maintains other ports
- added single fiber routing capabilities (pp.routing.add_fiber_single)
- added Component.copy()
- added basic electrical routing capabilities
- pp.routing.add_electrical_pads
- pp.routing.add_electrical_pads_top
- pp.routing.add_electrical_pads_shortest

- improve sparameters tutorial
- fixed some issues when using `x = x or x_default` not valid for `x=0`
- added tests for splitter_tree and splitter_chain

- get_netlist by default return a simpler netlist that captures only settings different from default. Full netlist component properties available with `full_settings=True`.
- limited pytest scope to netlist build tests to avoid weird side effects that move ports locations from test_component_ports
- sphinx==1.3.2 in requirements.txt

- `Component.get_netlist()` returns its netlist
- `Component.plot_netlist()` renders netlist graph
- `component_from_yaml` accepts netlist
- routing jupyter notebooks
- manhattan text can have cladding

- replaced hiyapyco with omegaconf (better YAML parser that can handle number with exponents 1e9)
- separated conf (important to be saved) from CONFIG that contains useful paths

- added link for [ubc PDK](https://github.com/gdsfactory/ubc) to README
- added a jupyter notebook tutorial for references and array of references
- added dbr and cavity components
- rotate is now a container
- adapted pp.pack from phidl as an easier way to pack masks
- Autoname also has now a build in cache to avoid having two different cells with the same name
- added type annotations