– Harmonize location output for warnings so that it is the same as for
  errors (i.e. more explicit about what is the line number and the column number)
– Remove obsolete stuff for delayed error messages that should have been
  removed in db1f6c29.
– Factorize some printing logic with ParsingDriver::undeclared_model_variable_error().
– Simplify WarningConsolidation class

 - at the end of the computation, don't display all preprocessor warnings but
   only their number
 - also display if there has been a MATLAB/Octave warning (counting their
   number does not seem feasible). Closes #181
 - add new "nowarn" option to disable all warnings. Closes #301