Added check_for_file()
Added check_for_file()

This function checks whether a file already exists in directory. Previously encapsulated within read_data(), now is separate in order to be reused for finding whether a 'GLEAM_snippet_..._deep' exists.

* Edits need to be made to read_data() to make it more streamline/efficient.
 - check_for_file should be referenced prior to entering read_data() loop. -> this will negate need for checking whether a file was found or not.
AstroRobin/GLEAM-RESID