Improve documentation on the Tau polynomials in the Tutorial 3

[navdeeprana]

The current documentation provides the code for the tau polynomials as

# Tau polynomials
tau_basis = xbasis.derivative_basis(2)
p1 = dist.Field(bases=tau_basis)
p2 = dist.Field(bases=tau_basis)
p1['c'][-1] = 1
p2['c'][-2] = 2

There is no explanation of the choice of tau_basis or the manual setting of the elements of p1 and p2. This should be clarified. I can do the work and open a pull request with some pointers on the explanation.

[navdeeprana]

After reading the documentation on the tau method and checking out other examples, I have come up with the following modified implementation, which is closer to the discussion of the tau method.

...
dx = lambda A: d3.Differentiate(A, xcoord)
d2x = lambda A: dx(dx(A))
tau1, tau2 = dist.Field(name="tau1"), dist.Field(name="tau2")
lift_basis = xcoords.derivative_basis(1)
lift = lambda A : d3.Lift(A, lift_basis, -1)
problem = d3.IVP([u, tau1, tau2], namespace=locals())
problem.add_equation("dt(u) - u - (1+1j*b)*d2x(u) + lift(tau1) + dx(lift(tau2)) = - (1+1j*c) * magsq_u * u")
problem.add_equation("u(x='left') = 0")
problem.add_equation("u(x='right') = 0")
...

Here's the output

I can modify the tutorial if needed.