datetime Series should raise on most numerical ops

[cpcloud]

obvs something like diff is fine, but i think sum/prod and most ufuncs should raise a TypeError maybe try the ops on the values attribute and if it doesn't raise continue

[jreback]

this is fixed for all numerical type ops, eg +, - etc (some make sense some don't)
you can prevent people from doing stupid things though

[cpcloud]

haha indeed. shouldn't other ufuncs raise though? i think it could be prevented since u know the dtype. maybe not for object arrays. + and friends raise but the corresponding reductions don't

[cpcloud]

there's an unconditional conversion to float64 in many ops in nanops.py. while practical for most purposes i think disallowing reduction ops on dates is the way to go. additionally probably good to only allow conversion to float if it's numeric and pass casting='safe' to astype, i.e., discourage the object dtype as much as possible.

[cpcloud]

this is a bit of an API change though, although currently there are no tests for summing over datetime64[ns] series/frames

[jreback]

This is ok for Frames, as they use _get_numeric_data, no dice for series though.....

In [1]: df = DataFrame(randn(10,2),columns=list('AB'))

In [2]: df['time'] = pd.date_range('20130101',periods=10)

In [3]: df
Out[3]: 
          A         B                time
0  0.655409 -2.119777 2013-01-01 00:00:00
1  0.326656 -0.208558 2013-01-02 00:00:00
2  0.284697  0.568025 2013-01-03 00:00:00
3  2.155815 -0.419372 2013-01-04 00:00:00
4  1.728936 -0.336092 2013-01-05 00:00:00
5  0.184181  0.642192 2013-01-06 00:00:00
6 -0.219383  0.794445 2013-01-07 00:00:00
7 -1.691735 -0.089012 2013-01-08 00:00:00
8 -1.388988 -1.407949 2013-01-09 00:00:00
9 -0.541974  1.472820 2013-01-10 00:00:00

In [4]: df.sum()
Out[4]: 
A    1.493613
B   -1.103279
dtype: float64

[cpcloud]

I should have this in by today. Should I start a new section in rls notes and a new what's new doc? You mentioned that there shouldn't be any more than doc prs until 11.1 is pushed