Create a dedicated workshop part.

Closes #12.

This provides a high-productivity workshop: it holds up to ten Kerbals and
gives a productivity factor of 5 (with a LOT of luck, up to 100, but
finding 10 Kerbals with 1,1,1 will be a challenge).

The part currently has no IVA, but I've already created #23 to cover this.
Rework the Kerbal productivity determination.

It turns out that parts without IVAs do not set kerbalRef, so use the
ProtoCrewMember data instead. Fixes #24.