allow cova_compute=0 with user-defined MCMC_jumping_covariance

(cherry picked from commit 35b1a685)

matlab/check_posterior_sampler_options.m

@@ -379,12 +379,14 @@ end
 % here are all samplers requiring a proposal distribution
 if ~strcmp(posterior_sampler_options.posterior_sampling_method,'slice')
     if ~options_.cova_compute && ~(options_.load_mh_file && posterior_sampler_options.use_mh_covariance_matrix) 
+        if strcmp('hessian',options_.MCMC_jumping_covariance)
         skipline()
         disp('check_posterior_sampler_options:: I cannot start the MCMC because the Hessian of the posterior kernel at the mode was not computed')
         disp('check_posterior_sampler_options:: or there is no previous MCMC to load ')
         error('check_posterior_sampler_options:: MCMC cannot start')
         end
     end
+end
 
 if options_.load_mh_file && posterior_sampler_options.use_mh_covariance_matrix
     [~, invhess] = compute_mh_covariance_matrix;