Add a benchmark report functionality

[mchalela]

Provide a function that computes time-benchmark stats for GriSPy methods. Something like the plots shown in the paper. This will allow us to easily compare the improvements introduced by new implementations. Time benchmark is the first priority, after that include a memory benchmark report.

API idea

The input should be the arrays of the parameter space over which the time stats should be computed.
For example:

import numpy as np
import grispy

# similar to Figure 4, second row.
Npoints = 10**np.arange(4, 8)
Ncells = 2**np.arange(3, 9)

# Compute the benchmark assuming a default configuration for the rest of the parameters
# Returns a custom pandas data frame with the stats
df = grispy.benchmark(Npoints=Npoints, Ncells=Ncells)  
df.plot(...)  # provide a custom plot method. This plot can be similar to those in the paper