FIX: Fix Main Thread Hanging from caget Call

[zdomke]

Main Problem

The DataInsightTool from #147 was causing the main thread to hang whenever a new curve was added. This came from a caget on a PV that didn't exist, resulting in the thread waiting the whole 5s timeout delay.

Main Fix

Move the caget call to a separate worker thread. The thread is spawned from the DataVisualizationModel. The model saves the description as an attribute and will prompt an update of the DataInsightTool. The meta_data label will be populated with a "Loading..." message while waiting for a CA response. This will really only be visible when the PV doesn't exist and is waiting to timeout.

Minor Fixes

• DataInsightTool.set_meta_data would throw errors if the user tried a caget call on a PV that doesn't exist. This function was trying to concatenate a NoneType with a str. Instead I am using str.join() to construct the meta data label text.
• The table was displaying the curve's unpopulated buffer if there was no real live data. This has been resolved.
• If the archiver tool sends an OK response with no data, an error was thrown. Now a warning message is logged stating that no data was received.
• In config.py I set the environment variable for NUMEXPR_MAX_THREADS if it is not already set because it not being set prints a warning on start up.

[zdomke]

I added the "Loading..." text to the label. I'm not entirely sure why the DIT is displaying filler data for the invalid curve though. That table should be empty. Will look into it.

This comment was resolved in commit a4f1743

[YektaY]

looks good to me. I have tested some of the changes here and they addressed the issues listed. great work!