Block certain user-specified names in the preprocessor

Allowing for a variable names like i may generate spurious crashes, particularly when used in conjunction with a steady state file as i is both the imaginary number and often the index variable of a loop. A similar case happens when someone names his data-file data or when he uses the mod-file name for the data-file. If the datafile for bkk.mod is named bkk.xls and the user only specifies data_file=bkk, Dynare will enter an infinite loop by calling bkk.m over and over again.