Change default filename created by savemacro option

The savemacro command line option appends -macroexp. It should be _macroexp because a hyphen is not allowed in Matlab file names. Consequently, running the generated file will result in

Error using dynare Dynare: argument of dynare must conform to MATLAB's convention for naming functions, i.e. start with a letter and not contain special characters. Please rename your .mod file.