Given a previously declared var_model, the var_expectation_model statement is
used to declare a way of forming expectations with this VAR (possibly using a
finite or infinite discounted sum). The var_expectation operator now takes a
single argument, the name of the var_expectation_model.

For the moment, this only works when the var_model is using equations
explicitly declared in the model block.

By the way, error out if the planner_objective statement is used two times.

- store objects whose persistence is not guaranteed (e.g. strings) as values
  instead of references (to avoid possible segfaults)
- on the contrary, always store the SymbolTable as a reference, since its
  persistence is guaranteed, and we don't want to copy it
- use pass-by-value in constructors whenever possible
- remove useless const keyword when passing by value

Most cases can be handled in C++11 by std::stoi() and std::stod().

For the macroprocessor ArrayMV<T>::print() method, use template
specialization (instead of a lexical_cast to detect between T=int and
T=string).

The latter accepts a std::string as argument.

- M and MEX files are now under +${MODELNAME}/
- bytecode and C source now under ${MODELNAME}/model/

In C++11, it is no longer necessary to put a space between two consecutive ">".

Performed using modernize-use-nullptr from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

Performed using modernize-use-equals-default from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-equals-default.html

Performed using modernize-use-auto from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-auto.html

Performed using modernize-redundant-void-arg from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-redundant-void-arg.html

Performed using modernize-pass-by-value from clang-tidy.

Manual intervention was needed to fix an issue in MacroValue.hh.
Also replace std::move by move, since we import the std namespace.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-pass-by-value.html

Performed using modernize-loop-convert from clang-tidy.

Manual intervention was needed in MacroValue.cc because of a variable name
capture issue.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-loop-convert.html

Consequently, change various function calls where tef_terms was empty, passing
an (empty) rvalue instead of an lvalue.