Avoid file overwrite

Function to check if a given file already exists. If a matching file is found, it creates a new filename that can be used to avoid overwriting. New file names are created by appending a sequence like '_001' to the file name. If earlier numerations exist already, the next unused number is used to allow saving file sequences, based on the same initial name. The starting value of the enumeration and the number of used digits are adjustable if needed.