Update Broombridge and chemistry library for UCC state preparation.

[guanghaolow]

Broombridge:

• Add UCC state preparation description
• Reduce number of mandatory fields that are not used.
• Clarify how various fields are used

Chemistry library:

• Add UCC state preparation method
• Minor restructuring to improve code reusability.

Specific tasks:

• In UnitaryCCWavefunction.cs of gulow/GenericLadderOperatos , we have the comment

// This class is for Fermion terms that are not grouped into Hermitian bunches. // Maybe need a stype for quantum state?

// For now, UCC is a subclass of MCF. It should eventually be a Hamiltonian
// + a WavefunctionSCF.

Other tasks: Modify Broombridge MCF to have a reference state that is not always the vacuum state https://github.com/microsoft/QuantumLibraries/pull/66#discussion_r275848514

[guanghaolow]

Addressed in #58,