OpenMP: Segmentation fault when writing to log/error files with multiple threads

[emilyviolet]

AMBiT sometimes crashes with SIGSEGV when multiple threads try to write to log/error files at once. These files are currently handled as standard C++ file streams and no mutex/locking, which is probably not thread safe.

[emilyviolet]

I have only encountered this as a consequence of issue #14, so surrounding writes to the error file with an OpenMP critical section might solve this without degrading performance once that issue is fixed (since we should no longer be hitting the error condition hundreds of times per run).