Incorrect pre-download filter for LIAMap

When filtering files to download from LIAMap application, files to downloads aren't filtered correctly.

We should:

• Select the pair with the best cover in the required time-range
• Don't look at fname_fmt.concatenation, but at fname_fmt.lia_product
• and then download the selected pair if need be.

The issue has been observed with the following scenario, while the last product (S1A_IW_GRDH_1SDV_20201221T044221_20201221T044246_035779_043013_4BE6) is not yet on disk

[DataSource]
download : True
first_date : 2020-12-01
last_date : 2020-12-30

[Processing]
calibration: sigma
tiles : 33NVB
fname_fmt.concatenation          : {flying_unit_code}_{tile_name}_{polarisation}_{orbit_direction}_{orbit}_{acquisition_stamp}_{calibration_type}.tif

See the log file.