Add implementation for downloading reanalysis data for downscaling and gapfilling of met variables

[gilbertozp]

Add implementation for retrieving data directly from ECMWF, extracting pixels from the downloaded ERA, apply unit conversions, and prepare/format data for running the downscaling process for a site now integrated into the ONEFlux pipeline.

Options for implementation of downloading:

• Downloading single pixel (or set of pixels) from ECMWF servers/APIs
• Downloading full archive and implementing service to provide extracted, corrected, and formatted files dedicated to downscaling

This issue replaces the portion of Issue #50 not covered in the inclusion of the downscaling code into ONEFlux (PRs #70 and #72)