Parameterized message type

[nossleinad]

We should consider a parameterized message type for FelNode, to increase performance when a message only contains instances of a single concrete type. The reason for this is to avoid conversions from Vector{PType} where PType <: Partition (and PType != Partition) to Vector{Partition} and Avoid containers with abstract type parameters.

[murrellb]

Good idea. We need to make sure that the implementation of this doesn't hurt generality though, or make it too annoying to manipulate the message (like adding an extra partition to the end of an existing message). Needs some thought.

[nossleinad]

There is a similar issue for BWMModel