Refuse that estimated parameters appear in the expressions defining the variance/covariance matrix of shocks

Closes #469
Refactor code for collecting variables appearing in expressions

- rename ExprNode::collectVariables in ExprNode::collectDynamicVariables
- new ExprNode::collectVariables: same as above, but without lag information
- remove ExprNode::findUnusedEndogenous: essentially redundant with the above)
- remove ExprNode::collectModelLocalVariables: idem
Add a warning when some exogenous are not used in the model
Forbid the use of trend variables in expressions outside the model block
Fix config of warnings in @dseries/cumsum tests
Fix for Octave (need to add extension of the m script).
Cosmetic change (use isdates routine).
Bug fix. Input must be numeric in the first place.
Bug fix (wrong handle).
Added new routine to test if a global variable exist in the base workspace.
Fixed bug in unitary test (due to a change of behaviour introduced last week by Houtan).
Fixed bug (isscalar does not assume that the input is numeric).
Adapted unitary test for octave.
Adapted unitary tests for octave.
Fixed bug (problem with dates selection in @dseries::subsref).
Cosmetic (removed commented line of code).
Revert cosmetic change of 4fa2d5b
bug fix: close unclosed file id, #438
Fix bug with DSGE-VAR introduced in f7cdc39f
Fix block/bytecode tests under Octave