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 differences