Make Climate Engine Metric Download asynchronous QgsTask

[KellyMWhitehead]

Climate engine download can take several minutes, especially for large downloads (time series, number of sample frames, number of metrics selected). Make this an asynchronous task so not to lock up qgis while the user waits.

1. Make the download an asynchronous QTask, and also the user to continue using the tool while the download proceeds in the background.

2. When the dialog closes, pre-select one dataset in the Climate Engine Metrics tab to help the user out. It's disconcerting that currently nothing happens when the dialog closes.

Implications:

• How does Climate Engine window react to the downloads and states.
• How does project tree react. Should we lock down sample frames from edits, deletions, etc.?
• Pre-Selecting a dataset to chart is fine, but what if user selects more than one metric to download, what if there are already metrics downloaded etc.? How does the asynchronous task impact this? Just want to make this a clean experience for the user.