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()