Shenfun

- There are now new classes for timestepping based on IMEX Runge Kutta.
- The Rayleigh Benard and Navier Stokes channel flow solvers have been completely rewritten using the new timesteppers
- A solver for micropolar Navier Stokes has been added

* It is now possible to configure how inner products are computed using `config['matrix']['assemble']`
* All matrix modules for orthogonal polynomials are much improved. Using `assemble` on subclasses of `SpectralMatrix`
* Introduced a new way to compute matrices using stencil matrices and the orthogonal basis. This is more efficient in terms of memory than regular Vandermonde type of quadrature.

* Introducing ultraspherical polynomials
* Much improved docstrings for all bases
* Improved description of boundary conditions
* Generic spaces and boundary bases that can be computed from the boundary conditions automatically. Hence we can now use any boundary conditions