Single and double timescale relaxation decorators. These can wrap
other decorators to introduce a timescale.

Use AR noise decorator

Results don't agree with experience with GENE-Tango

AnalyticFluxModel contains the flux calculation, for general p and q
values. ShestakovTestProblem fixes p=3, q=-2, defines a source and
has an analytic solution.

Input is now p and q, defaulting to 3 and -2 for previous case.
Analytic solution updated to work for arbitrary p,q

Modify the Shestakov model flux to more closely model the
characteristics of GENE turbulence simulations.

Modified the shestakov_time_dependent.py example

Rather than being classes which implement `get_flux` methods,
now functions which replace the `get_flux` method of given object.

The motivation is to combine the flux calculation method with the
test problem initial and boundary conditions, and exact solution
calculation. The goal is to make testing of different iterations
and boundary condition calculations simpler.