Changed extraction of amplifier gain.
Changed extraction of amplifier gain.

Made the FlowJoCollectorsEdition exception for extracting the
amplifier gain more conservative. Now checks '$PnG' keyword-value
first, and only if that doesn't exist does it try known exceptions
(e.g. 'CytekPnnG' keyword).

See #219, #220.

taborlab/FlowCal,taborlab/FlowCal