FiedlPropsManager::keys() list the FieldProperties needed by the
TransCalculator, but these cannot be queried the normal way as this
raises exceptions and results in a deadlock. Hence we use the new
funtionality to get also the unsupported ones, by passing true to
get_double_field_data.
They are attached to the cells as well and are now distributed
during CpGrid::loadBalance. Due to this change we also rename
FieldPropsDataHandle to PropsCentroidsDataHandle.