Main Program: Variable Declarations

In C functions, including main(), all variables must be declared by type before they are used. Often, it makes sense to initialize them to some default values upon declaration. Here, we make the default magnet 20$\times$20 in size, set the temperature to 1 (in dimensions of $J/k_B$), and provide some defaults fo the number of cycles (10$^6$) and sampling interval (10$^3$) in cycles (a cycle is one round of $N$ attempted flips). Variables for holding a couple of observables are initialized. Finally, the pseudorandom number generator from the GSL is declared.