Eliminate instance of Interactions from lattice model class

class Interactions is declared in a model class only for the namespace
reason.
src/lattice_numerical_constraint.cpp: assign meaning to empty dependence set

An empty dependence set is interpreted as dependence on all the
parameters except for t.
Generate lattice EWSB constraint
Allow dependence set to be omitted

The default value is {} which means all parameters except for t.