auto-design: relax min_gain restriction
auto-design: relax min_gain restriction

tolerate 3dB below min gain in amplifier selection
this is to allow selection of high power amps even if they are below min
gain

Signed-off-by: Jean-Luc Auge <ff93e793a4ae3350e3b728274a021e7856e8c6b3@orange.com>

Telecominfraproject/gnpy