Wrong behavior for nan converting from float to int

[TobyRoseman]

When changing the dtype of an SArray from float to int, nan get converted to very large number:

In [1]: import turicreate as tc

In [2]: sa = tc.SArray([1.0,2.0,float('nan'),3.0])

In [3]: sa
Out[3]: 
dtype: float
Rows: 4
[1.0, 2.0, nan, 3.0]

In [4]: sa.astype(int)
Out[4]: 
dtype: int
Rows: 4
[1, 2, -9223372036854775808, 3]

I'm not certain what the best behavior would be here; it seems nan is a floating point only representation. It would probably be best to turn the nan into None. Raising an exception is also probably reasonable.

[hoytak]

We should turn this into a None. It should be a simple fix.

[dhivyaaxim]

@TobyRoseman Please assign this ticket to me as I would like to try fixing it and contribute. Thanks in advance!