minor cleanup of lagrangian writing etc

• the writeLagrangianPositions switch is not registered (so local system/controlDict changes are ignored).
• the writeLagrangianCoordinate switch should now be dropped. No longer transition, things won't restart if coordinates aren't there.
• relocate geometryType enum from IOPosition to particle. This will make it non-templated.
• consolidate oldParticle struct into one place (eg, in particle) and rename as something like compat1706 so that its compatibility level is transparently documented via the API.

Optional: add -write-positions command-line option that forces writeLagrangianPositions on. This could be useful for things like reconstructPar, foamToVTK, foamToEnsight etc, when the user needs the information for external consumption, but only realized it after the simulation was complete.

@andy @Roger