Return the path to the final .mag file either for the complex or the fpi
    inversion.

def read_iter(use_fpi):
    '''Return the path to the final .mag file either for the complex or the fpi
    inversion.
    '''
    filename_rhosuffix = 'exe/inv.lastmod_rho'
    filename = 'exe/inv.lastmod'
    # filename HAS to exist. Otherwise the inversion was not finished
    if(not os.path.isfile(filename)):
        print('Inversion was not finished! No last iteration found.')

    if(use_fpi is True):
        if(os.path.isfile(filename_rhosuffix)):
            filename = filename_rhosuffix

    linestring = open(filename, 'r').readline().strip()
    linestring = linestring.replace('\n', '')
    linestring = linestring.replace('../', '')
    return linestring