Making Lotka–Volterra example more interesting.

Currently the solution to the Lotka–Volterra Chebgui demo
looks just like a sine and cosine. I have changed a couple
of constants to make the solution look more interesting
and asymmetrical.
Changes so that we still respect TRIGTECH discretization.
Merge branch 'fix-guiDemo-lotka' of github.com:chebfun/chebfun into feature-firstOrder-reform
Throw a meaningful error message in case we try to solve an integral
equation via first order reformulation.

Also, updating corresponding test so that it passes.
Finish setting up access control for treeVar methods.

Deleted an outdated test.
Removing files which were only relevant when developing the code.
Making the Lorenz IVP test a proper chebtest.
Introducing some more tests (currently failing).

Also throwing away test no longer needed.
Relaxing default absTol for IVPs.
Improving tests.
Ensuring we don't look at diffOrders and heights for trees that
actually are CHEBFUNs/scalars.
Changes in test structure, not going to test the outputs from
toFirstOrder separately.
Making the treeVar power method call the static bivariate() method.
Found out that a new property indicating whether expression has
terms in it makes the tree expansion more robust.
Further simplification of the expandTree method.
Correct sorting of output from first order conversion.
An big test of the first order conversion capabilities.
Adding documentation for new treeVar tree property.
Making t default name of independent variable when exporting IVPs.
We now print we're exporting to an m-file, rather than .m-file.