Do not require a PDK to be specified in the datasheet.

[jwise]

Some projects (i.e., PCB or other circuit simulation) do not have a PDK associated with them at all; they will use SPICE primitives, or other SUBCKTs, to do their work. For these projects, there is no need for a PDK key in the config; enable that behavior. This allows CACE to be used without a PDK installed at all.

[mole99]

Thanks for your contribution!
Looks good to me, just needs formatting and it's good to go.

I wonder for how long we will keep/maintain cace_compat, because the JSON format was used before the official release and probably not many datasheets are in this format. Also, there seem to be some deprecated options like foundry in it which I don't see in any of the recent datasheets. But that's for sometime later!

[mole99]

@jwise ping