adds working folder for unit test of forcing

examples/forcing_unit_test/README.rst

+Forcing unit test
+=================
+
+For this test, velocity field is initialized as a single mode.
+Numerical solution is compared with analytic solution for the different
+available forcing methods.
+
+
+**Linear forcing**
+
+initial condition
+
+.. math::
+    u(x,0) = U (e^{i\bar{k}\cdot x} + e^{-i\bar{k}\cdot x})
+    U \perp \bar{k}
+
+solution
+
+.. math::
+    u(x,t) = exp((-\nu\bar{k}^2 + \alpha)t) u(x,0)
+
+where :math:`\alpha` is the linear coefficient.
+
+**Fixed energy injection rate**
+
+initial condition
+
+.. math::
+    u(x,0) = U (e^{i\bar{k}\cdot x} + e^{-i\bar{k}\cdot x})
+    U \perp \bar{k}
+
+solution has same form as for linear forcing, but
+
+.. math::
+    U^2(t) = V^2 + (U^2(0) - V^2) exp(-2\nu\bar{k}^2 t)
+
+with
+
+.. math::
+    V^2 = \epsilon / (2\nu\bar{k}^2)