Implement more conventional behavior when opening non-existing file.

Instead of only printing a warning we now create a *named* and
empty buffer and change to the directory of the filename, so that
the file will be created in the desired location as soon as it is saved.

tools/lammps-gui/main.cpp

@@ -32,11 +32,6 @@ int main(int argc, char *argv[])
             printf("Usage: %s [-h|-help|<inputfile>]\n", argv[0]);
             return 1;
         }
-        if (!QFileInfo::exists(infile)) {
-            printf("Input file %s does not exist\n", infile);
-            printf("Usage: %s [-h|-help|<inputfile>]\n", argv[0]);
-            return 2;
-        }
     }
 
     LammpsGui w(nullptr, infile);