Automatically detected using clang-tidy with bugprone-reserved-identifier
check.

By the way, homogeneize the define identifiers in relation to camel case
convention.

Automatically detected using clang-tidy with modernize-use-nodiscard check.

By the way, remove the unused TrendComponentModelTable::getOrigDiffVar() method.

Creates a VarExpectationModelTable analogous to PacModelTable.

Ref. dynare#1837

Ref. Madysson/estimation-codes#5

Simple variable nodes are already correctly matched by
ExprNode::matchLinearCombinationOfVariables().

Also improve the related documentation.

There is no reason to keep this distinction.

Additionally, since the data structure is now symmetric with the MCE case,
unify this “h” parameter vector with the “α” parameter vector.

By the way, refactor the MCE case by merging two routines related to the Z₁ aux. var.
This restores the symmetry with the backward case, now that the latter also has
an aux. var. for the pac_expectation operator. Also store the aux. var. IDs in a
structure common to the backward and MCE cases.

Incidentally, the auxiliary endogenous variable representing Z₁ and created for
PAC MCE models no longer necessarily appear as the first auxiliary variable (so
this is effectively a revert of 64f55e4a).

As the name implies, this option allows contemporaneous variables on the RHS.

The A0 matrix for contemporaneous variables is added as a second (optional)
output to the generated var_ar.m file. Note that for reduced-form VAR, this
matrix will be the identity.

Also, the user is now allowed to write the VAR models in a more flexible form:
the LHS must still be a single variable, but the RHS can be an arbitrary
expression (as long as it is linear, obviously). Internally, the preprocessor
now uses derivation to compute the coefficients of the AR and A0. This change
applies to both reduced-form and structural VAR models.

Ref. dynare#1785

For a rationale, see http://www.sjbrown.co.uk/2004/05/01/always-use-explicit/