Since model local variables are now treated as temporary terms, they must be
marked as such even when option notmpterms is present.

Due to a limitation of the current implementation, this breaks syntaxes like
[ (i,j) ] (but not [ (2,j) ]; the problem only occurs when an array is
constructed by specifying as first element a tuple whose first element is a
variable name). Solving this problem requires an overhaul of the
macro-processor, with construction of ASTs at parsing time, and evaluation
later on (instead of doing on-the-fly evaluation).

Ref #5

Incidentally, replace the IS_* macros by inline functions.

This ensures a better isolation between the container and the containees.

- BTW, store them in a std::vector rather than std::list

- incidentally, fix issue in VariableNode::removeTrendLeadLag where expression
  sharing was possibly violated when creating a new VariableNode

- == and != have now lower priority than <= < >= >, for consistency with the
  Dynare modelling language (and incidentally C and C++, but not Julia).

- ^ has now higher priority and no associativity, for consistency with the
  Dynare modelling language (and usual arithmetic notation).

- & has now higher priority than |, and both have lower priority than + and -,
  but higher than inequality comparators. This is not the same as C and C++ (in
  which & and | are just above && and ||), but this allows for expressions such
  as "a|b == c" to have their most natural semantics (i.e. this will compare
  the union of a and b with c; the C/C++ priority level would have resulted in
  a type error).

Ref #5.

For consistency with the power operator for sets (see #5).