Added an option for the threshold level of the 0/0 generalized eigenvalue test.

The option is passed in the fourth input argument of mjdgges. If the
mex is called with less than four arguments, then the threshold level
takes its previous (hardcoded) default value: 1e-6.
Added default value for the threshold level of the 0/0 generalized eigenvalue test (1e-6).
Fixed bug related to the test on the number of input arguments.
Added input argument four (options_.qz_zero_threshold) when calling mjdgges.
Added fake argument to matlab's implementation of mjdgges.

Need to implement the test on 0/0 generalized eigenvalues later...
Adapted print_info routine.
Added qz_zero_threshold to check, stoch_simul and estimation commands.
Added documentatioon for the new option qz_zero_threshold.
Merge branch 'master' into experimental-mjdgges-threshold