worm_ising_2d.cpp segfault for large L

[tcompa]

worm_ising_2d.cpp fails for L=512.

• add a warning message for big systems
• move the declaration of big arrays (x, y, bonds, nbr) out of main()
• change bond_number() into a table of size Nx2 built like this: given site1, site2 and mu from 0 to 3, if mu<2 use index (site1,mu), else use (site2,mu)