Adds staticWhich and python3.WithPackages to ensure python at runtime

[augyg]

Kept having issues getting past an error "env: ‘python3’: No such file or directory" or matplotlb not found when depending on this haskell package in nix so this solves that by having the haskell derivation reference a python implementation with the appropriate python packages as well as "which" and then using the System.Which module to guarantee this executable at runtime over the env IO action.