Adds a new model option "linear_decomposition" that creates a block composed of the non-linear equations o the model

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

Incidentally, replace the IS_* macros by inline functions.

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

Otherwise boost::tuple overrides std::tuple.

Those were not automatically replaced by clang-tidy, I don’t know why…

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-noexcept from clang-tidy.

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

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

Manual intervention was needed to transform a NULL into nullptr in SymbolTable.cc.

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

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

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

More precisely, remove those variants where temporary_terms can be specified
without temporary_terms_idxs, in order to make clear that the latter is
expected. For situations where the tt_idxs are not needed (C, block MATLAB), an
empty map has to be explicitly given.

In the presence of MLVs, the temporary terms indexing was corrupted. The code
was using the implicit assumption that the ExprNodeLess ordering was giving the
same ordering as the temporary terms indexes ordering. But MLVs can be higher
in ExprNodeLess ordering than some other temporary terms, while they have the
lowest temporary terms index, hence the bug.

Fix this by no longer relying on the ExprNodeLess ordering, and rather use a
full map<ExprNode *, int> for ModelTree::temporary_terms_idxs. By the way,
simplify the code by removing a few useless data structures (e.g.
ModelTree::temporary_terms_idxs_*).

preprocessor: add model_local_variable declaration for declaring model local variables with tex names. Closes #563

preprocessor: add write_equation_tags option to write_latex_original_model and write_latex_static_model. closes #1431