Add a VESSEL entry class for base configuration files

[daves-ssu]

Something that could prove to useful is the addition of an VESSEL entry class for base configuration files. What this would do is allow for what I would call "dynamic objects", which would be essentially normal vessels but stuck to the ground and just allow for animated base objects like for example, the Vehicle Assembly Building of KSC's Launch Complex 39 that would have animated doors. And they would be loaded in every scenario as part of the base they're defined in.

Proposed layout of the entry is as follows:
DYNOBJ
FILE KSCVAB ;Path/name of cfg file
POS x y z ;Standard x y z base relative coordinates
ROT x ;Standard ROT parameter
END