49.3 Possible improvements to contrib_ode

These routines are work in progress. I still need to:

• Extend the FACTOR method ode1_factor to work for multiple roots.
• Extend the FACTOR method ode1_factor to attempt to solve higher order factors. At present it only attempts to solve linear factors.
• Fix the LAGRANGE routine ode1_lagrange to prefer real roots over complex roots.
• Add additional methods for Riccati equations.
• Improve the detection of Abel equations of second kind. The existing pattern matching is weak.
• Work on the Lie symmetry group routine ode1_lie. There are quite a few problems with it: some parts are unimplemented; some test cases seem to run forever; other test cases crash; yet others return very complex "solutions". I wonder if it really ready for release yet.
• Add more test cases.