Fix issue #374 (Crash with numpy version 1.15 and above (function np.product removed))

[osvalB]

Fixes Issue #374. Based on the numpy version, fit.py decides if np.prod or np.product should be used.

[tBuLi]

Thank you for supplying this fix!

LooseVersion does raise the following warning: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead. Perhaps you could switch to that recommended solution?

And the automated tests also give ImportError: cannot import name 'simps' from 'scipy.integrate', it seems that simps has been renamed simpson... Would you mind applying the same patch for that import error?

[osvalB]

Happy to help. I changed to 'packaging.version' and also changed 'simps' to 'simpson' according to the scipy version. Best, Os