Tutorial on checking energy preservation

[ranocha]

It would be nice to document the work done in #117 in a tutorial. We should probably also update the benchmarks section, e.g., using something along the lines of

In [4]: import time 
   ...: import pybs 
   ...: start_time = time.time() 
   ...: max_order = 10 
   ...: a = pybs.series.AVF 
   ...: b = pybs.series.modified_equation(a) 
   ...: result = pybs.series.energy_preserving_upto_order(b, max_order) 
   ...: end_time = time.time() 
   ...: print(result) 
   ...: print("", end_time - start_time, "seconds")