Dates handling inside Dynare statements was done in the Flex scanner using a
hack that would introduce some extra characters in the character stream (in
order to return a call to the “dates('…')” constructor to Bison). But this
would break the column location counter, because the latter would count
characters that do not exist in the user file.

This commit fixes it by implementing a more natural handling of dates, adding
the “dates('…')” constructor directly at the level of the semantic value.

This rule is obviously incorrect (and useless):
– it begins with an unescaped dollar sign, which can only match an end-of-line
  (so it makes no sense to have it at the beginning of the rule)
– it prints to standard output, which is obviously not the intended action

– New “heterogeneity_dimension” statement
– New option “heterogeneity” to “var”, “varexo”, “parameters”, “model” and
  “shocks” statements
– New “SUM()” operator in “model” block

See issue #1920

The symbol can be either input:
– as ⟂ in UTF-8 (codepoint U+27C2)
– or as _|_ in ASCII

The old syntax using the “mcp” tag is now obsolete, but kept for backward
compatibility.

Closes: #127

Closes: #124

The old name is still accepted, but will trigger a deprecation warning.

dr_block has been removed

Currently defaults to 0.

New homotopy options for perfect_foresight_{setup,solver} and perfect_foresight_with_expectation_errors_solver

Also remove homotopy_alt_starting_point option, and some options from
perfect_foresight_with_expectation_errors_setup.

Also renames the old static option

Related to dynare@2f236048

Related to dynare@3765ef77 (dynare!2094)

Ref. dynare#1856

By the way, remove other unnecessary calls to that constructor.

Incidentally, forbid exogenous deterministic variables in “endval” blocks.

For use with perfect_foresight_with_expectation_errors_setup.

New option “constant_simulation_length” to “perfect_foresight_with_simulation_errors_solver” command