operator: add density as material type

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
Operator: get access to cell center material averaging coordinates

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
fix and improvemnt in getting cell center coordinate for material averaging

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
operator: change how to average material to allow for overloaded cylindrical handling

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
ports: calculate/export time domain voltages and currents

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
Engine interface: check for valid operator and engine

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
Operator: new method to get corresponding engine

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
critical fix: use new Operator->GetEngine for engine interface setup

Bug fix: engine interfaces for multigrids were using subgrid operator,
but main grid engine...

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>