acb_ode, numerical solution of linear ODEs with polynomial coefficients

[mezzarobba]

This is a preview of the code for solving linear ODEs with polynomial coefficients I've been working on, now using the recently merged version of the gr_ore_poly module.

The main feature at this stage is evaluation of fundamental matrices of differential operators within their disk of convergence (in other words, transition matrices for individual Taylor steps), including tail bounds and support for regular singular points.

The API is not final, and the implementation is still buggy. It is also missing a lot of tests, alternative algorithms for some subtasks, optimizations, etc. Additionally I'd like to add a few more high-level features before declaring it ready for review, if only to simplify testing and check that the low-level API makes sense.

@fredrik-johansson @simonbrandhorst @Tobias271828 Any feedback is welcome, in particular about the API. Also feel free to tell me if there are missing features you'd like me to prioritize.

[mezzarobba]

Note that @rburing has an implementation of more or less the same algorithm at https://gitlab.inria.fr/ricardo-thomas.buring/d-finite-fun, but

• over generic rings,
• for nonsingular first-order systems (vs regular singular equations here),
• with a focus on computing truncated series (vs evaluating the sum).