Pennylane-rigetti

* Added ability to return multi-qubit expectation values to all devices

* Added variance support

* Replaced dense matrix expansion with a tensordot implementation

* Added support for `samples`

* Readout Error Mitigation using Operator Estimation

* Updates the plugin to work with PyQuil 2.13 and PennyLane 0.6

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New features

* Updated to support PyQuil v2.9.0 and PennyLane v0.4.0. Note that PennyLane-Forest now requires PennyLane>=0.4

* The observable `qml.Hermitian(A, wires)` can now be measured on an arbitrary number of wires. That is, you can now pass a matrix A of size `[2**N, 2**N]` which acts on `N` qubits (i.e., `len(wires)==N`).

* Adds support for the `qml.var()` measurement return type provided by PennyLane v0.4.

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* Using any of the available lattices at any given time, instead of a specifically named one

* updated minimum pyquil version requirement to 2.7

* added support for pyQVM

* removed 2.4 changes

* Updating compiler urls

* Replacing old lattice with new

* pyqvm working without the compiler

* Updating compiler urls

* Replacing old lattice with new

* added build and documentation badges

* Remove the LocalQVMCompiler from the tests