Moved much code into PlotLayer base class.
Moved much code into PlotLayer base class.

apeeyush/Data-Analytics-CODAP,concord-consortium/codap,concord-consortium/codap,apeeyush/Data-Analytics-CODAP,concord-consortium/codap,apeeyush/Data-Analytics-CODAP,concord-consortium/codap,concord-consortium/codap