The interaction operator that is the sum of the generators of the

specified fermionic simulation gates.

The gates are specified as a dictionary whose items are (indices, gate), where

* indices is a tuple of ints specifying the modes on which the gate
* gate is one of type
  - float, which is interpreted as a constant, regardless of the
  - cirq.ZPowGate, which is interpreted as a "linear" fermionic
    simulation gate,
  - openfermion.InteractionOperatorFermionicGate.

n_modes The number of modes.
gates The gates.

The interaction operator.