Add new preprocessor option minimal_workspace

Matlab has a limit on the number of variables allowed in the workspace. To prevent problems with this limit, introduce new preprocessor option minimal_workspace that when used, suppresses writing the parameters to a variable with their name in the workspace. That is, instead of

M_.params( 1 ) = 0.999;
beta = M_.params( 1 );

only write M_.params( 1 ) = 0.999; Also set options_.minimal_workspace to 1 (default: 0) so that we can suppress the call to dyn2vec in stoch_simul to prevent duplicating simulated variables in the workspace. See the email of Stéphane from 26/05/15