Rework how scanlines are 'aged'.

Realised that it takes 2.5s for a full revolution of the radar scanner, so normal
minimal age is 3 (s). Add control so users can change this between 1 (for testing)
and 12 (seems plenty...)

Fixes bug #36.

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# Committer: Kees Verruijt <kees@kees-mbp.verruijt.lan>
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# On branch master
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#	modified:   src/br24radar_pi.cpp
#	modified:   src/br24radar_pi.h
#	modified:   src/br24radarcontrol.cpp
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