Fix bug in acor

@maudelejeune : the chunck of chain (argument X) passed to python_acor (in tools.py) is passed by ref thus the function, while computing the auto-correlation is actually changing the values of the chains array. As a result the parser dumps wrong chains values in the final 'npy' files. I've added a quick and dirty workaround making a copy of the array. Also added a workaround to nan values for the correlation length in get_autocorr_len.

Let me know if you see inconveniences (I may have missed something). If not I'll merge this evening.