- add pad_pitch to `resistance_sheet`
- enable multimode waveguide in straight_heater_meander
- add `grating_coupler_elliptical_arbitrary`
- add `grating_coupler_elliptical_lumerical` using lumerical parametrization
- rename `grating_coupler_elliptical2` to `grating_coupler_circular`. rename `layer_core` to `layer`, `layer_ridge` to `layer_slab` for a more consistent parametrization of other grating couplers.
- add Component.add_padding