If a DSGE-VAR is estimated, check that the user do not try to estimate/calibrate correlations between structural innovations and that there is no measurement errors. Closes #521.
Cosmetic. Renamed DsgeVarLikelihood as dsge_var_likelihood.
Added explicit error message for case 52 in print_info (DSGE-VAR models).
Added comments in mod files (tests/dsge-var).
Various changes in tests/dsge-var mod files.

+ Changed comments.
+ Use new Dynare's interface for specifying the version of numgrad.
Added references for DSGE-VAR model. Fixed reference for Abramovitz and Stegun.