Unrequested variable in extractions that span date of variable added to dataset

[douglatornell]

Discovered during investigation of issue #43.

When an extraction date range includes the date on which a variable was added to a dataset, the added variable is included in the extracted dataset regardless of whether or not it is in the list of requested variables. Known occurrences of this are:

• HRDPS-2.5km-operational.yaml profile for percentcloud variable added to dataset on 8-Jul-2017
• HRDPS-2.5km-operational.yaml profile for variables LHTFL_surface, PRATE_surface, and RH_2maboveground added to dataset on 6-Dec-2018

This issue is demonstrated for the 1st of those cases by the extraction config:

dataset:
  model profile: HRDPS-2.5km-operational.yaml
  time base: hour
  variables group: surface fields

dask cluster: salish_cluster.yaml

start date: 2017-07-01
end date: 2017-07-31

extract variables:
  - precip

extracted dataset:
  name: HRDPS_hour_precip_surface
  description: Hour-averaged surface precip rate extracted from HRDPS 2.5km operational product
  dest dir: /tmp/

The extracted dataset contains variables precip as requested, and percentcloud, with the latter having NaN values prior to 2017-07-08.