read_output function for NetCDF.

[ali-ramadhan]

Just merging this so I can work on cleaning up the examples.

Couple of NetCDF related issues I hit:

• NetCDF.jl seems to be missing some features and isn't really being maintained (See Writing values into unlimited dimension variable JuliaGeo/NetCDF.jl#62 about saving time values and scale_factor, add_offset and _FillValue JuliaGeo/NetCDF.jl#39). Maybe it's worth switching to NCDatasets.jl which takes a more data frames approach to NetCDF and is actively maintained and grew out of bugs that weren't being fixed in NetCDF.jl. Unfortunately we're choosing between two relatively young packages. An alternative would be to use the much more mature netcdf4-python but I'd rather not have to use PyCall...
• We might need to decide on some standard field/variable names. Standard field names for NetCDF output. #91
• Output might need to take into account cell-centered and face-centered fields How to deal with cell-centered and face-centered fields in NetCDF output? #92

Reopens #31

[glwagner]

Output needs to be arbitrary. We may need to perform on-line analysis and output the result (example: turbulent dissipation rate, time-averages, slices of fields, point values, etc).

We should design an additional interface for Fields. The type of the field indicates the coordinates on which the field is defined, so we should design an interface that uses that information.