Replace the static char buffers at the beginning of the SetConvergence_History() routine in output_structure.cpp

When monitoring the forces on a large number of markers, the output won’t fit in the buffer(s), and this causes memory issues: the code either hangs, has a buffer overflow, or segfaults depending on the compiler, etc. As a temporary fix, the monitoring_coeff[] buffer has been increased, but this should be remedied asap.
