Commit f431b1c breaks using the indexes function on python lists

[Zachary Hofmann]

Commit f431b1c breaks using the indexes function on plain python lists

specifically this snippet from peak.py:

if np.issubdtype(y.dtype, np.unsignedinteger): raise ValueError("y must be signed")

which raises this error:

File "/usr/lib64/python3.5/site-packages/PeakUtils-1.0.1-py3.5.egg/peakutils/peak.py", line 32, in indexes AttributeError: 'list' object has no attribute 'dtype'

[Lucas Hermann Negri]

Thanks. While I wasn't thinking in supporting lists (and the function doc did state that), adding an extra check made it work again with plain lists.

Thanks again.

[Zachary Hofmann]

Thanks to you as well, this module is great!