ENH-19629: Adding numpy nansun/nanmean, etc etc to _cython_table
- [x] closes #19629 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

As per the issue, here's proof of solution:
```python
Python 3.6.4 |Anaconda, Inc.| (default, Jan 16 2018, 12:04:33) 
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> import numpy as np
>>> pd.Series([1,2,3,4]).agg(np.sum)
10
>>> pd.Series([1,2,3,4]).agg(np.nansum)
10
```

Where should I add tests for these changes? I wasn't sure on the best fit and how to most effectively test. Also, is a whatsnew entry needed here? If yes, any guidance on what it should be?