refactored extract.nc function to find closest lat/lon

I refactored this to take advantage of the ncea function, which can find the closest point, e.g. 

This still needs to be tested

```sh
ncea -d lat,44.4 -d lon,-88.1 infile.nc outfile.nc
```

_notes_
* if lat / lon are decimals, they are latitude and longitude
* if lat/lon are integers, they are assumed to be indices
* ncea will average over space (and time) if two values / a range of values are given.