add basic Field doc and demos

Common/DefaultOptions.h

Parser/Gmsh.tab.cpp

Parser/Gmsh.yy.cpp

benchmarks/2d/Square-01.geo

benchmarks/2d/Square-Attr1.geo

benchmarks/2d/Square-Attr2.geo

benchmarks/2d/Square-Attr3.geo

benchmarks/2d/Square-Attr4.geo

benchmarks/2d/Square-Emb.geo

benchmarks/2d/TieAnchor520.geo

benchmarks/2d/naca12_2d.geo

benchmarks/2d/t7.geo

benchmarks/2d/wing-splines.geo

benchmarks/2d/wing.geo

benchmarks/2d/world.geo

benchmarks/3d/Cube-Attr-01.geo

benchmarks/3d/Cube-Attr-02.geo

benchmarks/3d/Revolve2-Attr.geo

benchmarks/3d/Torus-Attr.geo

benchmarks/3d/cube.geo