Zero delays as nulls

[rjendoubi]

Flights without delays have no values in certain "delay" columns in the CSV.
When those are loaded, they are treated as null values in Hive, and that persists to the ORC tables as well.
However, this then causes problems for certain visualizations created at the Data Viz part of the lab.

We should either

• Fix the CSV to have zeros in those columns instead of no value, or
• Update the CTAS queries to explicitly put zeros in those columns IF NULL, or
• Include instructions on how to deal with this in CDV

Opening this to track, will aim to come back with more details and/or a PR.