New readme and other file changes

Readme has a little more intro information and four inline plots, from
images folder.

In calculus.jl - default Hilbert for IntervalDomains now goes to
JacobiWeightSpace(-.5,-.5…

In Evaluation.jl - added a periodic boundary condition

In systems.jl - I realized Sheehan that you are doing some changes with
blkdiag, but commenting out diagm caused the Floquet notebook to crash.
If you’re not planning on keeping diagm, I’ll change the notebook.

In JacobiWeightOperators.jl - The constants were correct with the
ChebyshevSpace, so when moving to Jacobi, the fix is to multiply
everything by (d.b-d.a)/2, as you calculated for
the log case.

In Clenshaw_evaluate.m - I don’t think this is necessary any longer.

In OperatorTest.jl - less trivial periodic ODEs.