strip down test_Parseval file

bfps/test/test_Parseval.py

 #! /usr/bin/env python
 
-import os
 import numpy as np
-import h5py
 import sys
 
 import bfps
 from bfps import DNS
 
-import matplotlib.pyplot as plt
-
 def main():
     niterations = 10
     nlist = [16, 32, 48, 24, 64, 12]