DSS2 — Simplify representation of Logical Time

The use of valuetypes for time in the DistributedSimulation IDL require
a user of this cap to provide matching implementations of LogicalTime
and LogicalTimeInterval for both the client federate and RTI. This
would require coordinated efforts to match implementations (perhaps in
different implementation languages) for federates and the RTI. Since a
user of the IDL interface will only be writing code on the client
(federate) side, this approach adds unnecessary complexity. It is
recommended that the abstract valuetype for logical time and logical
time interval be replaced with a double in the section headed “Logical
time, time stamps, and lookahead”, and that the IDL also be changed
analogously, with the forward reference to LogicalTimeInterval being
removed and the definitions of abstract valuetypes LogicalTimeFactory
and LogicalTimeIntervalFactory being removed.

Simplify by replacing the abstract valuetype for logical time with a double.