Squashed merge of particleInteractions up to
commit e7cb5bcf0315c359539ef1e715e1d51991343391

Have weight=7 on cells to be refined and balance. Then after refinement
there will be perfect balancing. Similar for layer addition.

When faceZones are present one has to decide whether to have zone faces
only on the edge of cellZones or also freestanding.
The setting for minMedialAxisAngle was corrected since the calculation
has also been corrected.

Moved the snappyHexMesh tutorials to the appropriate solver directories and created links.

Created a runnable tutorial out of the iglooWithFridges case to demonstrate the buoyantBoussinesqSimpleFoam code.