Added a quench example using the staggered_magnetization observable

import sys, os

from pathlib import Path

sys.path.insert(1, str(Path.home()) + "/.xacc")

from qcor import *

import numpy as np

import matplotlib.pyplot as plt

# Heisenberg quench model input

problemModel = qsim.ModelBuilder.createModel(ModelType.Heisenberg, {'Jz': 0.8,

                                                       'h_ext': 0.2,

                                                       'ext_dir': 'X',

                                                       'num_spins': 8,

                                                       'observable': 'staggered_magnetization'})

print(problemModel)

# Run TD workflow:

workflow = qsim.getWorkflow(

      'td-evolution', {'dt': 1.0, 'steps': 20})

result = workflow.execute(problemModel)

# Plot the result:

t = np.linspace(0, 20, 21)

y = result["exp-vals"]

axes = plt.axes()

axes.plot(t, y)

axes.set_xlim([0,20])

# axes.set_ylim([0,1])

# Save the plot to a file:

os.chdir(os.path.dirname(os.path.abspath(__file__)))

plt.savefig("staggered_magnetization_plot.pdf")  

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        qsim, "QuantumSimulationModel",

        "The QuantumSimulationModel captures the quantum simulation problem "

        "description.")

        .def(py::init<>());

        .def(py::init<>())

        .def(

            "__str__",

            [](qcor::qsim::QuantumSimulationModel &self) {

              std::stringstream ss;

              ss << "{ observable: " << self.observable->toString() << "}";

              return ss.str();

            },

            "");

    // ModelBuilder bindings:

    py::class_<qcor::qsim::ModelBuilder>(