Qmat

Some small fixes + testing on the `Jumper` type generators

Added two new $Q_ \Delta$ generators :
- `Jumper` : diagonal coefficients allowing 2 order jump per SDC sweeps
- `FlexJumper` : mix between the `Jumper` and `MIN-SR-FLEX` generators, allowing order jumps while still maintaining high numerical stability

🎓 _Inspired from the work of [Joscha Fregin](https://www.mat.tuhh.de/home/jfregin_en) and [Eugen Bronasco](https://www.unige.ch/~bronasco/)_

Main rework of the documentation, fixing some aspects on readthedocs

Added derivation matrix generation for the `LagrangeApproximation` class (since it's a bit free). Second order still need the correct formula ...

- addition of fourth order stiffly accurate RKM with 13 thanks to brownbaerchen
- addition of LDU based QDeltaGenerator with 1107c603d3d2e6103074752c68f2d5d28e0a1aae

Addition of a new RK method with 12 thanks to brownbaerchen