fix issue #79

[MoZhonglin]

Setting np.set_printoption's threshold param to np.nan is no longer allowed and should use sys.maxsize instead:

/anaconda3/lib/python3.7/site-packages/numpy/core/arrayprint.py in _make_options_dict(precision, threshold, edgeitems, linewidth, suppress, nanstr, infstr, sign, formatter, floatmode, legacy)
91 # forbid the bad threshold arg suggested by stack overflow, gh-12351
92 if not isinstance(threshold, numbers.Number) or np.isnan(threshold):
---> 93 raise ValueError("threshold must be numeric and non-NAN, try "
94 "sys.maxsize for untruncated representation")
95 return options

ValueError: threshold must be numeric and non-NAN, try sys.maxsize for untruncated representation

[MoZhonglin]

The issue has been fixed in exercises100.ktx and all the notebook files are regenerated.