Issue 1014 Possibly attempt object -> float64 coercion in to_sparse
closes #1014

I've added a check for dtype=object and try to coerce it to a float in frame.to_sparse

hey interesting idea here, can you fix all the issues that I pointed out and then ping us when they're fixed so we can discuss?
 @jdowns why is your data object (that are really floats) in the first place? that actually is somewhat hard to do, unless you are casting it. or is the intention to filter out non-numerics first?
 @jreback this was the problem from issue #1014.  The person who opened the ticket had that problem.  I'm not sure how it came up.
 I see that now...thanks...

ok...glad you have an interest in to_sparse! FYI it changed quite a lot in 0.13. So in theory, you can support multiple dtypes in the sparse objects, so this might not be strictly necesary. And as I said above its hard to actually have non-object numeric data.

This is probably accomplished by just a `self.convert_objects(convert_numeric='force')` to force conversion (after you have detected an object-dtype, or possible, `self._get_numeric_data()`. Really depends on what the purpose is.
 @jreback I did have the issue of getting objects when I expected floats once.  I was reading a fixed width data file that had dashes to make it look better to the human eye.  It was easy to skip that line, but I could see somebody inadvertently getting a data frame like that and just wanting to go on with their computation. Judging by the example in issue #1014 I think the use case is similar. 
 @jreback the problem is that you'd need to do convert_objects and then cast to float, right?
 @jreback Yes, that's a solution to the problem I encountered. 
 `convert_objects(convert_numeric='coerce')` will do all of this; no casting necessary it will cast as needed and turn non-numeric into nan (if you pass force), works on series and frames. astyping is just for changing types (e.g. float64 -> float32), it _will_ in some cases do the same thing but not generally
 What if it returns int instead?
 it will attempt downcasting as needed (e.g. there's a defined conversion com._possibly_downcast_to_dtype), in essence if its a float and compares equal to a casted version and no nulls (and its not a datetime/bool/timedelta) then its casted).

This is user initiaded operation (most of the time) because its not exactly cheap to do this
 @jreback Because this is an expensive operation, does it make sense to add a flag in the parameters to try convert_objects?
 I don't think this should automatically convert; rather you can check if their are non-valid dtypes

right now I think no non-floats are allowed (though in theory it should work).
 closing

ithis should raise/warn on object dtype not auto-convert
