Enable read_array function to return multiple arrays

[Suizer98]

I am modifying the read_array function inside nco.py as below:

   def read_array(self, infile, var_names):
        file_handle = self.read_cdf(infile)

        for var_name in var_names:
            try:
                # return the data array for each variable
                result[var_name] = file_handle.variables[var_name][:]
            except KeyError:
                print("Cannot find variable: {0}".format(var_name))
                raise KeyError

        return result

The usage for my case would be:

variables = ["longitude", "latitude", "time"]
result = nco.ncks(
    input="sample.nc", options=["-v " + ",".join(variables)], returnArray=variables
)

lon = result["longitude"][:]
lat = result["latitude"][:]
...

so we don't have to call returnArray=[] one by one.
I remember the native nco commands extract multiple variables such as:
ncks -v latitudes,longitudes
can support extracting multiple variables from netcdf file.
If this kind of actual feature is already existed, feel free to address how should I use/call it, in case I missed it.

[czender]

Your suggestion looks reasonable to me. Would you please submit a PR that implements it, so we can see if it passes all the tests?

[Suizer98]

Your suggestion looks reasonable to me. Would you please submit a PR that implements it, so we can see if it passes all the tests?

Okay sure, will do one PR for this