File_search() to return given files from certain path and subdirectories

Hi guys,
Take 2!
Here is a quick addition to the util module.  The new function, file_search(), returns a list of file names in a given path (and its subdirectories) which satisfy a certain pattern, e.g. *.txt.  There is a similar function in IDL which solar physicists would be used to using and is very handy for finding files if you don't know the nature of the directory you're searching.  It is a very short function consisting of a list comprehension.  But it is a complicated list comprehension and required two imports.  Combining this into a single function is very handy and usable.
Let me know of any comments.
Cheers!
Nobody except you knows why these numbers exactly have been selected as indices. See @rhewett's proposal on how to avoid them. It makes the code easier to understand and perhaps more stable if you use the functions from `os.path` instead of reading "magic slices" of the string.