Support for DIS entity dead-reckoning

[met4000]

Story

As a programmer using disco
I want to be able to query the dead-reckoned position of an entity
So that I can use a more accurate approximation of the entity position instead of an old last-known position

Context

In order to get the location of an entity, the Disco EntityStateStore application can be used to get the position information from the last-seen EntityStatePDU.
In situations where the DIS entity is expecting simulations to use dead-reckoning, the entity's heartbeat rate will likely be very low.
For fast-moving entities, this causes the position retrieved from the EntityStateStore to have a significant margin of error from the actual entity position.

Acceptance Criteria

Once complete, there should be a Disco application that can:

• Maintain dead-reckoning-related information of specified entities (either indefinitely or for a longer time than EntityStateStore does)
• Provide the position of an entity at a given time using the dead-reckoning model specified by that entity
• Respect the 'Frozen' flag on entities (no dead-reckoning should be performed)

Support for other DIS objects that support dead-reckoning (e.g. designators, articulated parts) is not in scope of this issue, and if required should build upon changes that provide the features in this issue.

[met4000]

All requirements met by #94. Will be closed once that PR has been completed.