Enable support for gf180mcuC

[donn]

WIP, cannot be merged until open_pdks and volare are updated.

---

• Add gf180mcuC configs for {APU, PPU, SPM} ~ Clean up some leftover variables ~ STD_CELL_LIBRARY now an optional environment variable- open_pdks config files are now responsible for setting the default ~ to_tcl.py updated to reflect that ^ ~ Tweak mergeLib behaviour because gf180mcuC behavior is kinda nuts ~ Better error checking for LEF file ~ Add ability to just set METAL_LAYER_NAMES in the PDK