Automatically detected by clang-tidy using modernize-use-equals-default check.

HANK support: modifies the driver file writing process to introduce a `state_var` field in the `M_.heterogeneity(i)` structure for each heterogeneity dimension `i`

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