folders sub-folders

This functions goal is to return a list (cell array) of names of sub-folders located under user defined folders list. The input should be a a cell array of parent directories names. The function also supports input of a single directory name string.
Alternatively, the user can choose the folders using the OS explorer- by enabling the 'flagGUI' input.
Function properties:
- It is not recursive but iterative.
- It does not change the current folder (not using "cd" command).
- It uses "ls" function, which is supposed to be faster than "dir" function.
- According to some measurements I've made it runs faster than the alternatives.
- I also believe it is clearly written and documented, so it should be easy to understand and maintain.