Rename model classes

[vibraphone]

Now that the old model classes have been removed, some renames should happen:

• ModelEntity → Model
• GroupEntity → Group
• InstanceEntity → Instance

Also, we have been asked to consider

• BridgeSession → ModelSession or KernelSession or just Session?
• Bridge → SessionImpl or SessionCore or SessionState
• Cursor → Item

on the grounds that Bridge and Cursor might mean something to developers but not end users.

[vibraphone]

The final (but probably not completely comprehensive) list of class renames is

• Cursor → EntityRef
• GroupEntity → Group
• InstanceEntity → Instance
• ModelEntity → Model
• Bridge → Session
• BridgeSession → SessionRef
• BridgeIOJSON → SessionIOJSON
• BridgeIO → SessionIO
• BridgeRegistrar → SessionRegistrar
• RemusRemoteBridge → Session
• RemusBridgeConnection → RemusConnection
• RemusBridgeConnections → RemusConnections
• BridgeExodusIOJSON → SessionExodusIOJSON