Updates to meshing a full annulus

* Commit fixes a problem with the flagging of tangential edges where the
  annulus is "glued" together; re-inserted edges were all markes as inner.
* Removes old version of MeshAnnulus() that had CRISSCROSS hardcoded.
* Updates MeshGenTest to test all four meshing flavours.
* Introduces hollowFlag enum in MeshInfo class for setting in a mesh generator
  flagInnerBoundary, flagInterior, flagOuterBoundary for closed and hollow
  domains such as the full annulus and the thick spherical shell. MeshAnnulus()
  now used these.
* Extends documentation.
* Also fixes clipping of CRISSCROSS and CROSS picture for MeshRectangle()