Return the path to the final rho*.mag file from the tomodir.

def read_iter():
    '''Return the path to the final rho*.mag file from the tomodir.
    '''
    filename = 'exe/inv.lastmod'
    linestring = open(filename, 'r').readline().strip()
    linestring = linestring.replace('\n', '')
    linestring = linestring.replace('../', '')
    return linestring