Random Number Generator in SDA.

documentation included in mod_par_zig.f90

SDA has two possible implementations for generating random numbers:

the same method as that was used in SDA 6 : this was kept for comparison with the original code during developement, but it cannot be used within an OpenMP environment
a Ziggurat thread safe method (see Marsaglia, G. & Tsang, W.W., J. Statist. Software, 5, 8, (2000).) : a method developed for OpenMP, each thread has its own sequence of random numbers ( mod_par_zig.f90 ).

The implementation used is chosen at compile time. If OpenMP is activated the Ziggurat method is chosen, otherwise the original SDA 6 routine is used.
The thread-safe functions are declared in maths_thread_safe.f90 .
The original implementation is in randgen.f . The critical section is commented in the source code.

The Ziggurat method allows full parallelisation of sda_2proteins runs, and parallelisation of BD moves in sdamm simulations ( see make_bd_move_sdamm.f90 ).

Both implementations are initialised in read_input_file.f90 ( function initialize_random_generator ).