Fix PyLammps regression after output.cpp change

    runs = []

    columns = []

    in_run = False

    current_run = {}

    for line in lines:

        if line.startswith("Memory usage per processor"):

        if line.startswith("Per MPI rank memory allocation"):

            in_run = True

        elif in_run and len(columns) == 0:

            # first line after memory usage are column names