Make plot layers & inner subplot layers consistent

[Mattriks]

• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• makes the layer.statistics field of plot(layer(...)) and the inner layers of Geom.subplot_grid() consistent (fixes Stats in Geom.subplot_grid #1516)
• updates Stat.violin as needed
• an open question from this PR is what should be in the layer.statistics field of the outer layer of layer(Geom.subplot_grid(layer(...))). Note that this should not necessarily be the same as
  the statistics field of the inner layers, which are clearly defined by this PR.
  E.g. the outer layer of Geom.subplot_grid could contain statistics which modify aesthetics such as :xgroup and :ygroup, but
  no such statistics have been developed yet for Gadfly.

Example

(now p2 shows same behaviour as p1 from OP in #1516):

p2 = Geom.subplot_grid(Stat.smooth, Geom.ribbon)
p2.statistics   # StatisticELement[]
p2.layers[1].statistics # StatisticElement[Stat.smooth()]

• Note in this PR, the fields p1.statistics and p2.statistics have not been deprecated, because they may be useful for developers.

[Mattriks]

Regression testing showed 1 difference: testscripts/auto_enumerate.jl shows Geom.line layer but no Geom.bar. Will investigate.