Support for PDKs without TAP / ENDCAP cells

[smunaut]

Description

Some PDKs like iHP don't need tap cells or endcaps.
Currently the variable FP_WELLTAP_CELL / FP_ENDCAP_CELL and FP_TAPCELL_DIST are strictly required for OL2 to recognize a PDK, even if you have a custom flow that doesn't include the OpenROAD.TapEndcapInsertion step at all.

Ideally they shouldn't be required and if you don't have them specified, then the flow should use CutRows instead of TapEndcapInsertion step.

Proposal

No response

[donn]

Ugh so this one's a bit of dilemma because making variables that all steps should expect to have as an option would be an API break.

For OpenLane 2.2, I recommend you just set them to dummy values. For 3 (EOY), we'll break the API and fix this.