adding simple example using numpy linear algebra (#143)

examples/simple_numpy.py

@@ -0,0 +1,30 @@
+import numpy as np
+from optlang import Model, Variable, Constraint, Objective
+
+# All the (symbolic) variables are declared, with a name and optionally a lower
+# and/or upper bound.
+x = np.array([Variable('x{}'.format(i), lb=0) for i in range(1, 4)])
+
+bounds = [100, 600, 300]
+
+A = np.array([[ 1, 1, 1],
+              [10, 4, 5],
+              [ 2, 2, 6]])
+
+w = np.array([ 10, 6, 4])
+
+obj = Objective(w.dot(x), direction='max')
+
+c = np.array([Constraint(row, ub=bound) for row, bound in zip(A.dot(x), bounds)])
+
+model = Model(name='Numpy model')
+model.objective = obj
+model.add(c)
+
+status = model.optimize()
+
+print("status:", model.status)
+print("objective value:", model.objective.value)
+print("----------")
+for var_name, var in model.variables.iteritems():
+    print(var_name, "=", var.primal)