v0.4.1

Updated isotope pattern calculation algorithm to IsoSpec (http://pubs.acs.org/doi/abs/10.1021/acs.analchem.6b01459):

• faster than previously used Ecipex (http://pubs.acs.org/doi/full/10.1021/ac500108n)
• KissFFT dependency has been dropped
• user can set desired total probability instead of simply thresholding: isotopePattern(desired_probability=..., threshold=0)