Fix/default constructor initializer list

A default constructed FoamGrid should be a valid (but empty) grid. The list of entites was previously uninitialized but this data member is accessed in several interface functions.

The entity list is now properly initialized. Level 0 always exists, but is empty (empty entity lists) when the grid is default constructed. An empty level index set is also created for level 0.

Improves test by adding an empty grid test.