New BC and SpatialParams interfaces

This is related to #350 (closed), !274 (closed) and !276 (closed).

Can we finalize the new interfaces for the boundary/initial conditions and spatial parameters? I currently find the following for the BC/IC:

BoundaryTypes boundaryTypes(Element, SubControlVolume)
BoundaryTypes boundaryTypes(Element, SubControlVolumeFace)
BoundaryTypes boundaryTypesAtPos(GlobalPosition)

PrimaryVariables dirichlet(Element, SubControlVolume)
PrimaryVariables dirichlet(Element, SubControlVolumeFace)
PrimaryVariables dirichletAtPos(GlobalPosition)

PrimaryVariables neumann(Element, FVElementGeometry, ElementVolumeVariables, SubControlVolumeFace)
PrimaryVariables neumannAtPos(GlobalPosition)

PrimaryVariables source(Element, FVElementGeometry, ElementVolumeVariables, SubControlVolume)
PrimaryVariables sourceAtPos(GlobalPosition)

PrimaryVariables initial(SubControlVolume)
PrimaryVariables initialAtPos(GlobalPosition)

int initialPhasePresence(Vertex, int, GlobalPosition)
void initialPhasePresenceAtPos(GlobalPosition)

We should make the initial... ones consistent with the other ones. Like this?:

PrimaryVariables initial(Element, SubControlVolume)
PrimaryVariables initialAtPos(GlobalPosition)

int initialPhasePresence(Element, SubControlVolume)
int initialPhasePresenceAtPos(GlobalPosition)

Can we consider this as final?

What about the spatial parameters?