new ComposeVars()
This is a wrapper on top of multiple variables which are of same type but they are shown in the same axis.This is
required in case of range plots or ohlc plots where one plot is mapped to multiple measure fields. At that time,
we need to create a composed variable from multiple variables. This class just wraps them into one variable instance
and provides methods to get the type and other common functionalities which can be performed on a simple variable
instance.