Return start_dir or the parent dir that has the s2 marker.

    Starting from the specified directory, and going up the parent
    chain, check each directory to see if it's a base_dir (contains
    the "marker" directory *s2*) and return it. Otherwise, return
    the start_dir.

def discover_base_dir(start_dir):
    '''Return start_dir or the parent dir that has the s2 marker.

    Starting from the specified directory, and going up the parent
    chain, check each directory to see if it's a base_dir (contains
    the "marker" directory *s2*) and return it. Otherwise, return
    the start_dir.

    '''
    if is_base_dir(start_dir):
        return start_dir
    pcl = start_dir.split('/')  #path component list
    found_base_dir = None
    for i in range(1, len(pcl)+1):
        d2c = '/'.join(pcl[:-i])
        if (d2c == ''):
            d2c = '/'
        if is_base_dir(d2c):
            found_base_dir = d2c
            break
    return found_base_dir