Update trellis traceback function to assign HMM if unassigned
Update traceback_path.cpp function to check for NULL model

Functions that require HMM model will now check and return
warning if model is not defined.
Fix: options incorrect assignment of OPT_DOUBLE

When a double option was specified it was getting assigned to the OPT_INT and then not accessible.
Changed it to correctly access the OPT_DOUBLE
Added options to threshold Posterior table output

Added option -threshold to command-line options as a OPT_DOUBLE
Added function void print_limited_posterior(…) to output limited posterior -only output lines where value is greater than -threshold
Changed perform_posterior function to call print_limited_posterior if threshold is provided.