tropo_pyaps3: use larger SNWE local files if available

[yunjunz]

Description of proposed changes

This PR searches existing local grib files with larger SNWE extent than the input required SWNE, uses that if more local files exist, thus, could avoid re-downloading from ECMWF as much as possible.

• add get_larger_snwe_in_local_files() to find and locate the local files with larger SNWE extent than the input original ones, and use that if more local files exist.

• move many tiny functions as sub-functions inside the main ones, to improve the code readability (simpler code logic flow)

Reminders

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