The preprocessor now writes all the symmetric elements in the "hp"
matrix (derivatives of the hessian w.r.t. parameters), for consistency with all
other derivatives output.

Previously it was only writing one of the two symmetric elements, when indices
of endogenous were different.

Also, no longer compute two times symmetric elements in derivation w.r.t.
parameters at order 2, for consistency with derivation w.r.t. endogenous.
It is therefore now necessary to duplicate them in the output to keep behavior
unchanged.

When creating the sparse matrix (in MATLAB or C mode), since storage is in
column-major order, output the first column, then the second, then the third.
This gives a significant performance boost in use_dll mode (at both compilation
and runtime), because it facilitates memory accesses and expression reusage.

Closes #13

This is the first step towards computing k-order derivatives.

Ref dynare#217, #10

New options "mexext" and "matlabroot" are introduced, so that the preprocessor
knows where to find MATLAB and which architecture to compile for.

Only recent gcc is now supported. A set of optimization flags is used so that
compilation goes reasonably fast on large models.

Consequently, options "msvc", "mingw" and "cygwin" have been removed.

It is therefore now possible to write:

  static_model = static_cast<StaticModel>(dynamic_model);

In particular, it is necessary to turn back DataTree::AddVariable() into a
non-virtual method, because it is called from DataTree's constructor. Enforcing
the absence of leads/lags is now done using a new boolean DataTree::is_static.

Take advantage of the new copy constructor for handling
PlannerObjectiveStatement more elegantly.

Unfortunately it is not possible to implement *move* constructor / assigment
operators, because the reference ExprNode::datatree is not mutable.

This facilitates switching variable types on the fly. In particular, this
allows removing the hack in DynamicModel::updateAfterVariableChange() that way
basically recreating all the nodes after the type change.

The syntax makes clearer the distinction between a function call and an
initialization. Also, it prevents bugs due to implicit type conversions.

Closes: dynare#1315

This mimicks the structure of M-functions (though the logic for filling the
temporary terms vector is a bit different).

This change implied a modification in the way we compute the checksum in case
of block decomposition (the temporary terms for the C output are not correctly
computed in that case).

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.

ensure that trend_variable found is actually a trend variable as declared in the trend_component_model statement