Check if given directory to see if it is usable by s2.

    Checks that all required directories exist under the given 
    directory, and also checks that they are writable.

def verify_dir_structure(full_path):
    '''Check if given directory to see if it is usable by s2.

    Checks that all required directories exist under the given 
    directory, and also checks that they are writable.

    '''
    if full_path == None:
        return False
    r = True
    for d2c in PREDEFINED_DIR_NAMES:
        #if d2c == "s2":
        #    d2c = ".s2"
        cp2c = os.path.join(full_path, d2c)  #complete path to check
        if not os.path.isdir(cp2c):
            r = False
            break
        else:  #exists, let's check it's writable
            if not os.access(cp2c, os.W_OK): 
                r = False
                break
    return r