Merge pull request #237 from komatits/devel

added option to broadcast the database between simultaneous runs for identical mesh and model (but different source and/or receiver).
added test of size(array) > 0 before broadcasting them (arrays of size 0 are legal in Fortran for some reason, but can create problems in the call to our broadcast routines because we call them with array(1) as argument). Swapped two "use module" statements in src/specfem3D/fault_solver_*.f90 because the first used the second, which led to an internal compiler error with some versions of GNU gfortran.