Find workaround for warnings at Dynare startup under Octave

When Dynare starts under Octave, one gets:

warning: function /home/sebastien/dynare/unstable/matlab/+pac/+bgp/get.m shadows a built-in function
warning: called from
    dynun at line 4 column 5

warning: function /home/sebastien/dynare/unstable/matlab/+pac/+bgp/set.m shadows a built-in function
warning: called from
    dynun at line 4 column 5

This is the consequence of an Octave bug.

A partial workaround was implemented in 4c0b2e8c, but it does not suppress early warnings.

A better solution should be found.