Allow to install a single PDK variant

[mole99]

Description

Now that we have --pdk and --pdk-family as parameters, perhaps --pdk could be used to install just one or more PDK variants and --pdk-family to install the whole family as usual.

Proposal

This would probably require separating the data for the individual PDK variants in the assets.

[donn]

I think this would require a major rearchitecture of Ciel. Additionally as we start using symlinks this may complicate things even further... Not sure.