That class is meant to contain data about named functions (e.g. sin,
tan...) in one place: their name, their number of children and a pointer to
a builder. The derived class corresponding to each such function
contains a private instance (m_functionHelper) and a getter.
The previous parser is removed, along with unecessary
constructors (used by the previous parsers).
Replaces the name() method in ExpressionNode subclasses.
When necessary, the createLayout and serialize methods in thoses
subclasses were moved to their .cpp file.
