Missing differential equation functionality in Octave

[kolmanthomas]

With my custom Octave build running on CI (see #28, you can take a look at the CI test results there), we are now down to 20 failing tests in the @chebfun directory. I've identified some DE functionality missing in Octave that would need to be worked around (or added to Octave) in order to get rid of some of these failures:

• The function deval hasn't been implemented in Octave. I saw that someone had an implementation of the function here: https://rouxph.blogspot.com/2019/07/une-fonction-deval-pour-octave.html (I hope your French is good @cbm755!). Not sure if this is sufficient, also we may need to ask properly since I don't see an explicit license for the code.
• ode113 is not implemented in Octave. Perhaps we can swap this with another ODE solver, but I haven't really looked into it much.
• vdp1.m is not implemented in Octave. I believe this is just a demo function (see here: https://www.mathworks.com/help/matlab/ref/odextend.html, especially the line "Solve the van der Pol equation with using ode45. The function vdp1.m ships with MATLAB® and encodes the equations.").
• ode15s is in Octave, but needs to be enabled. In order to have it in Octave, the user needs to have built with the SUNDIALS library. There's currently some issues with certain configurations not being able to build with SUNDIALS (see https://octave.discourse.group/t/improving-mpi-detection/6484). You can't really do anyhting about this @cbm755, just wanted to clarify what was happening here.