A node for missing data

[jonathanjfshaw]

In timeflux/timeflux_ui#9 @mesca thought that a node for dropping NaNs could have a place in core.

I've also found circumstances where I've wanted to provide an empty dataframe (instead of None) and where I've wanted to add in NaNs (because I wanted the indices to exist).

I propose a node that can handle various degrees of missing data:

• providing an empty dataframe instead of None
• provide a dataframe with an index (or additional indices) but no values
• drop NaNs
• replaces NaNs with values (e.g. zeros)