RBC potential bug: lattice coordinates being produced by implicit truncation

[rupertnash]

I've now disallowed implicit conversions Vector3D -> Vector3D (you can still do this explicitly of course). This has thrown up a number of cases where floating point Vector3D's had been implicitly truncated to integer Vector3Ds, particularly around positions.

I don't know if any of these are bugs.

[rupertnash]

Examples:

hemelb/Code/redblood/CellArmy.h

Line 342 in 9ddd354

auto const iter = globalCoordsToProcMap.find(LatticeVector{barycenter});

hemelb/Code/unittests/redblood/CellForceSpreadTests.h

Line 80 in 9ddd354

return latDat->GetSite(LatticeVector{center}).GetForce();