Suppress warnings during the models spectra calculation

[Sheshuk]

Sometimes running the calculation of the neutrino spectra from the models produce runtime warnings, even when the calculation is within the valid range. These warnings are of no concern to user and just flood the logs

Example:

import numpy as np
import astropy.units as u
from snewpy.models import ccsn

m = ccsn.Bollig_2016(progenitor_mass=11.2<<u.M_sun)
spectra = m.get_initial_spectra(m.get_time(), np.linspace(0,100,51)<<u.MeV)

produces following warnings:

.../snewpy/python/snewpy/models/base.py:305: RuntimeWarning: divide by zero encountered in log
  np.exp(np.log(L) - (2+a)*np.log(Ea) + (1+a)*np.log(1+a)
.../snewpy/python/snewpy/models/base.py:305: RuntimeWarning: invalid value encountered in subtract
  np.exp(np.log(L) - (2+a)*np.log(Ea) + (1+a)*np.log(1+a)
.../snewpy/python/snewpy/models/base.py:306: RuntimeWarning: divide by zero encountered in divide
  - loggamma(1+a) + a*np.log(E) - (1+a)*(E/Ea)) / (u.erg * u.s)

This makes no problem in the final spectra/flux array, because all the NaN/inf values get filled as zero in the final result, but the warnings are a nuisance and should be suppressed.