Add support for reading entire dataset

[charleskawczynski]

It seems ncread requires a variable argument name. It would be convenient, and more efficient in some cases, to be able to read the entire dataset rather than specified variables.

[meggart]

Would you imagine something like this?

function ncread(filename::String)
    nc = NetCDF.open(filename) 
    d = Dict(k=>NetCDF.readvar(v) for (k,v) in nc.vars)
    NetCDF.close(nc)
    d
end

i.e. just returning a Dict containing all the variables or would you rather return a full struct with all the metadata + data?

[charleskawczynski]

Yes, I think that would be a fine interface.