Add access to the scales at draw time using a new `layer_params` object#3170

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paleolimbot:layer-layer-params

Commits on Feb 19, 2019

implement layer params that live in the layer
paleolimbot
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Commits on Feb 22, 2019

make layer_params a ggproto object, modify get_scale to take layout as an arg, move layer_params to get assigned in setup_layer().
paleolimbot
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remove ref to scales list, fix layout scale getting
paleolimbot
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Commits on Mar 1, 2019

add test and news item in prep for PR
paleolimbot
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