eljost/pysisyphus: Nix support and many robustness fixes

Addition Installation of pysisyphus & dependencies like QC codes using Nix thanks to @sheepforce DLC coordinate weighting On-the-fly primitive coordinate rebuilding if the current set becomes invalid/ill-defined Arbitrary precision calculation of internal derivatives using mpmath Revamped RFOptimizer Added second type for linear bends (LinearDisplacement) and OutOfPlane coordinate (both are not used yet) RMSDs and barrier heights are now reported after "endopt" runs if possible Bugfixes Generated derivatives for dihedrals fail at 0° and 180° ... disabled them for now pinv is now used with proper rcond value ... Miscellaneous Removed redund_v2. LinearBends are now used by default if necessary Primitive internal setup & update was moved outside the RedundantCoord class into separate function, which should facilitate reuse Generated code for primitives is now verified against finite difference