Openfermion

Upgrade cirq compatibility.

What's Changed
* [Docs] Format math expressions by rmlarose in https://github.com/quantumlib/OpenFermion/pull/738
* Adding mention of OpenFermion-FQE to the frontpage by babbush in https://github.com/quantumlib/OpenFermion/pull/740
* Added functionality to compute one-norm of qubit/majorana Hamiltonian from molecular integrals by Emieeel in https://github.com/quantumlib/OpenFermion/pull/725
* Updates linux CI to use 18.04 by MichaelBroughton in https://github.com/quantumlib/OpenFermion/pull/747
* Updates fqe include path for site rendering by MichaelBroughton in https://github.com/quantumlib/OpenFermion/pull/746
* Upgrade to cirq 0.12 by tanujkhattar in https://github.com/quantumlib/OpenFermion/pull/748
* Version bump by ncrubin in https://github.com/quantumlib/OpenFermion/pull/749

New Contributors
* tanujkhattar made their first contribution in https://github.com/quantumlib/OpenFermion/pull/748

**Full Changelog**: https://github.com/quantumlib/OpenFermion/compare/v1.1.0...v1.2.0

Creating official release from March 6th v1.0.1 release of OpenFermion associated with commit ( c22551fbb8f7965c0d3fd0fc119ff2f8b569917a).

This release is for the stable 1.0 version of OpenFermion. Significant changes from the 0.11 release have been made. The most notable changes are listed below.

Features:

MajoranaOperator <-> FermionOperator with Jordan-Wigner + Bravyi-Kitaev options, get_majorana acts like get_fermion operator.
Is_contextual for confirming if a Hamiltonian is contextual
SymbolicOperator coefficients are sympy.Expr objects
Partitioning schemes for measuring fermionic 1- and 2-RDMs
Functionality for the Hartree-Fock project
Spatial -> spin-orbital integrals
Extended RPA
Constructing a pure 2-body (reduced Hamiltonian) from 1-body and 2-body InteractionOperator.
Construct antisymmeterized 2-electron integrals + coulomb + exchange matrices
Verified phase estimation utilities

Organization:

Merging of OpenFermion-Cirq into OpenFermion
Reorganize package (620) (new submodules: chem, linalg, circuits, functionals, transforms)
Sample data is integrated with package for testing.
Reorganize utils
Deprecation of Readthedocs as the documentation host. Docs are now generator by tensorflow-docs and hosted at https://quantumai.google/openfermion

Starting with this version, Python 2 is no longer supported. Other notable changes and additions:
- Functions to partition qubits into subsets for efficient measurement of Pauli strings: `openfermion.binary_partition_iterator`, `openfermion.partition_iterator`, `openfermion.pauli_string_iterator`.
- Added `structure` argument to `openfermion.geometry_from_pubchem`. It is recommended to set this to '3d' to ensure correct structures are returned (see https://github.com/quantumlib/OpenFermion/pull/544).
- `openfermion.get_number_preserving_sparse_operator`.
- `openfermion.prony`.