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

Uses:
– structured bindings
– std::map::contains and std::map::at
– std::boolalpha and std::noboolalpha

std::fill takes a range that is open on the right bound (i.e. [first, last) and
not [first, last]).

[skip ci]

Closes: #128

It was previously in case-sensitive alphabetical order (i.e. ASCII order).

Ref. #128

– use std::pair instead of std::tuple in internal storage
– make Environment::getFunction() symmetric with Environment::getVariable()

Ref. #125

Rather use the data member which already exists.