Add various methods to frame, series, cell, etc.

Unfortunately this is a bit of a hodgepodge of additions.

- Adds a `to[CC]` converison method to Series, so we can easily convert to SCala collections where necessary (eg. `series.to[List]`).
- Adds some support for summary statistics back in via `framian.stats.ops`.
- Adds a `reduceFrameWithCol` method, which reduces the frame, but includes the values from a single col in the reducer for each column.
- Adds `zipMap` back to `Cell`.
Yeah - it seemed a little out of place, given most other things, including median, return NA in this case. If the data is not available, then I think saying the mean is not available is valid. Does that make sense to you?