Ameliorate Warnings, PDN Generation, and the default design (#2066)

~ Reimplemented the default serial/parallel multiplier to be a bit less cryptic and eliminate linter warnings; and added a proper testbench
~ Reimplemented deprecated variable translation behavior to account for the situation where a deprecated variable's value does not match the default for the new value, which causes a crash
~ Slightly improved warning for designs having been black-boxed during STA
~ PDN Generation Updates
    ~ Renamed `DESIGN_IS_CORE` to `FP_PDN_MULTILAYER` with translation behavior
    ~ PDN generation will now always attempt to extend to core ring (which is inert if core rings do not exist)
    ~ Fixed bug where `FP_PDN_MULTILAYER` being set to `0` would attempt to create a core-ring on two layers anyway
~ IR drop now prints a warning if `VSRC_LOC_FILE` is not provided
- Removed deprecation behavior for `GLB_RT` variables - it's been over a year (>=6 mo as promised)