Siepic-ebeam-pdk

New components:
- Bragg grating PCell, with parameters from a file
- broadband 50/50 splitter for 1310 and 1550 operation: ebeam_splitter_swg_assist_te1310.gds
- SWG_assisted_Strip_WG PCell
- Multibox Bragg Grating
- 1310 nm grating coupler, ebeam_gc_te1310_8deg.gds

New functionality:
- Waveguide: support for multiple waveguide CML and models

Fixes:
- waveguide pin width rounding error, which sometimes caused false Functional Verification errors
- other minor fixes

- contradirectional coupler
- several fixes
- added parameter for simulation accuracy
- Photonic Wirebonding cell
- updates to CML
- PCM spiral data

- fixes to CDC

- contra-directional coupler PCell compact model generation using external python environment.

- updates to libraries to fix compatibility issues with KLayout 0.26

- new components to mature library (with CMLs)
++ sub-wavelength assisted coupler
++ adiabatic y_splitter 1x2

- metal pin snapping (merge pull request from S. Preble)

- new components to developers library
++ multiperiod bragg gratings
++ dual period contra-directional couplers
++ Sub-wavelength contra-directional couplers

- updates to scripted layout examples with additional examples

- added cross-section viewer