Add verbose argument for PowerSpectrumStatistics.load()

[echaussidon]

In order to avoid multiple line when loading one powerspectrum in several process, I propose to add verbose argument here:

pypower/pypower/utils.py

Line 190 in e44fe1a

def load(cls, filename):

[adematti]

I'm a bit reluctant to add an option that would be redundant with the logger level.
Wouldn't the best option (also to avoid unneccessary IOs) to load power spectra from a single process, then do comm.bcast(power, root=0)?

[echaussidon]

Agree to track the I/O accross all the processes.

However it is quite heavy when reading 1000 power spectra to compute the covariance Matrix for instance.