Add feature to return dataframe rather than numpy array

[kevindougherty-noaa]

In get data, add an input as return_df as a boolean. If True, return the dataframe.

[aerorahul]

Is there a reason, not to always return a dataframe?
What is the advantage of returning just the variables?

[kevindougherty-noaa]

@aerorahul You bring up a good question. I think the original idea when creating this interface was to be made for those not familiar with python. That way, all you need is two lines to get an array of a specific variable that you want (i.e. omf, oma, observation, hofx). Returning just the dataframe would remove a lot of lines from diags.py. It may require some changes in other scripts, but I am indifferent about keeping the current way or changing to just return the dataframe. What do you think? A decision should be made before I start diving in with developing regional DA monitoring scripts any further.